Your FREE AP Chemistry Practice Test 2026 – 240+ Q&A

AP Chemistry Practice Questions

1. A chemist needs to convert 88 grams of carbon dioxide, with a molar mass of about 44 grams per mole, into an amount in moles. What is the correct result?
   • 0.5 mole
   • 2 moles
   • 4 moles
   • 44 moles

   Correct answer: 2 moles

   2 moles is the correct answer. Dividing the 88-gram sample by the molar mass of 44 grams per mole yields 2 moles, applying the standard relationship that amount in moles equals mass divided by molar mass.

2. An element's mass spectrum displays a single sharp peak rather than several peaks. What does this most directly indicate about the element?
   • It exists as essentially one isotope
   • It has many isotopes of equal abundance
   • It is a transition metal
   • It cannot be ionized

   Correct answer: It exists as essentially one isotope

   It exists as essentially one isotope is the correct answer. A single peak in a mass spectrum means nearly all atoms share the same mass, so the element is effectively monoisotopic with no significant lighter or heavier forms.

3. Boron has two naturally occurring isotopes, and its average atomic mass listed on the periodic table is closer to 11 than to 10. What does this average value reveal about the isotopes?
   • Both isotopes are equally abundant
   • The lighter isotope is far more abundant
   • The heavier isotope of mass 11 is more abundant
   • Boron has only one isotope

   Correct answer: The heavier isotope of mass 11 is more abundant

   The heavier isotope of mass 11 being more abundant is the correct answer. Because the weighted average is pulled toward 11, the isotope of mass 11 must make up the larger fraction of naturally occurring boron atoms.

4. Which set of quantities is needed to calculate the number of molecules in a measured mass of a pure substance?
   • Only the temperature and pressure
   • The molar mass and Avogadro's number
   • The boiling point and density
   • The atomic radius and electronegativity

   Correct answer: The molar mass and Avogadro's number

   The molar mass and Avogadro's number is the correct answer. Mass is divided by molar mass to get moles, and moles are multiplied by Avogadro's number to find the count of molecules in the sample.

5. In a photoelectron spectrum of phosphorus, how many distinct peaks would be expected based on its ground-state electron configuration?
   • Two peaks
   • Three peaks
   • Four peaks
   • Five peaks

   Correct answer: Five peaks

   Five peaks is the correct answer. Phosphorus has electrons in the 1s, 2s, 2p, 3s, and 3p subshells, and each of these five occupied subshells produces a separate peak at its own binding energy.

6. A photoelectron spectrum shows two peaks whose heights are in a ratio of 1 to 3. Which pair of subshells could produce this ratio?
   • A filled 2s subshell and a filled 2p subshell
   • Two different s subshells
   • A filled p subshell and an empty d subshell
   • A 1s subshell and a 2s subshell

   Correct answer: A filled 2s subshell and a filled 2p subshell

   A filled 2s and filled 2p subshell is the correct answer. A full s subshell holds two electrons and a full p subshell holds six, giving a 2 to 6, or 1 to 3, ratio of peak heights in the spectrum.

7. Effective nuclear charge experienced by a valence electron is best described as which of the following?
   • The total number of neutrons in the nucleus
   • The net positive charge felt by an electron after accounting for shielding
   • The total charge of all electrons combined
   • The energy required to form a bond

   Correct answer: The net positive charge felt by an electron after accounting for shielding

   The net positive charge felt by an electron after accounting for shielding is the correct answer. Effective nuclear charge equals the actual nuclear charge reduced by the screening effect of inner electrons that lie between the nucleus and the electron in question.

8. Why do inner core electrons shield valence electrons from the full nuclear charge more effectively than other valence electrons do?
   • Core electrons have more energy than valence electrons
   • Core electrons carry a positive charge
   • Core electrons lie between the nucleus and the valence electrons
   • Core electrons are larger in size

   Correct answer: Core electrons lie between the nucleus and the valence electrons

   Core electrons lying between the nucleus and the valence electrons is the correct answer. Their position closer to the nucleus lets them block, or screen, much of the nuclear attraction, while valence electrons at similar distances shield one another only weakly.

9. For a multi-electron atom, which subshell within the same principal energy level is generally lower in energy?
   • The p subshell is always lowest
   • The d subshell is lowest
   • The s subshell because it penetrates closer to the nucleus
   • All subshells in a level have identical energy

   Correct answer: The s subshell because it penetrates closer to the nucleus

   The s subshell because it penetrates closer to the nucleus is the correct answer. Greater penetration lets s electrons experience a higher effective nuclear charge and less shielding, lowering their energy relative to p and d subshells in the same shell.

10. Which ground-state electron configuration correctly represents a neutral magnesium atom, which has 12 electrons?
    • 1s2 2s2 2p6 3s2
    • 1s2 2s2 2p6 3s1
    • 1s2 2s2 2p6 3p2
    • 1s2 2s2 2p8 3s2

    Correct answer: 1s2 2s2 2p6 3s2

    1s2 2s2 2p6 3s2 is the correct answer. Filling subshells in order of increasing energy places two electrons in 1s, two in 2s, six in 2p, and the final two in 3s, accounting for all twelve of magnesium's electrons.

11. When sulfur gains two electrons to form a sulfide ion, how does its electron configuration change?
    • It loses electrons from the 3p subshell
    • It removes its 3s electrons
    • It becomes identical to that of a noble gas with a filled 3p subshell
    • It adds electrons to the 4s subshell

    Correct answer: It becomes identical to that of a noble gas with a filled 3p subshell

    Becoming identical to a noble gas with a filled 3p subshell is the correct answer. Sulfur has the configuration ending in 3p4, and gaining two electrons fills the 3p subshell to 3p6, matching the stable configuration of argon.

12. Among the elements lithium, beryllium, boron, and carbon, which is expected to have the highest first ionization energy?
    • Lithium
    • Beryllium
    • Boron
    • Carbon

    Correct answer: Carbon

    Carbon is the correct answer. Moving left to right across the second period increases effective nuclear charge and decreases atomic radius, so carbon, farthest right of the four, holds its outer electron most tightly and has the highest first ionization energy.

13. A neutral atom and its cation differ in radius primarily because the cation has which characteristic?
    • Fewer electrons, reducing electron-electron repulsion
    • More protons than the neutral atom
    • A larger principal energy level
    • More neutrons in the nucleus

    Correct answer: Fewer electrons, reducing electron-electron repulsion

    Fewer electrons, reducing electron-electron repulsion is the correct answer. Removing electrons to form a cation leaves the remaining electrons more strongly attracted by the unchanged nuclear charge, so the cation is smaller than its parent atom.

14. According to Coulomb's law, how does increasing the magnitude of the nuclear charge affect the attraction between the nucleus and a fixed electron?
    • The attraction decreases
    • The attraction stays constant
    • The attraction increases
    • The attraction becomes repulsive

    Correct answer: The attraction increases

    The attraction increases is the correct answer. Coulombic force is directly proportional to the product of the interacting charges, so a larger positive nuclear charge pulls a given electron more strongly toward the nucleus.

15. Why does the energy required to remove an electron from a hydrogen atom in its ground state represent the largest ionization energy for that single electron compared to removing it from an excited state?
    • Excited electrons are closer to the nucleus
    • The ground-state electron is in the lowest, most tightly bound energy level
    • Excited atoms have more protons
    • Ground-state electrons carry less charge

    Correct answer: The ground-state electron is in the lowest, most tightly bound energy level

    The ground-state electron being in the lowest, most tightly bound level is the correct answer. An electron in the ground state sits closest to the nucleus at the lowest energy, so removing it requires more energy than removing an electron already raised to a higher, less tightly held level.

16. A compound is found to contain 2 moles of hydrogen atoms for every 1 mole of oxygen atoms. What is the empirical formula consistent with this ratio?
    • $\ce{HO}$
    • $\ce{H2O}$
    • $\ce{HO2}$
    • $\ce{H2O2}$

    Correct answer: $\ce{H2O}$

    $\ce{H2O}$ is the correct answer. An empirical formula expresses the simplest whole-number ratio of atoms, and a 2-to-1 ratio of hydrogen to oxygen corresponds directly to the formula $\ce{H2O}$.

17. Which statement best explains why fluorine has a smaller atomic radius than oxygen, even though both are in the same period?
    • Fluorine has fewer electrons
    • Fluorine has one more proton, increasing the pull on the same shell
    • Fluorine has an additional electron shell
    • Fluorine has more neutrons

    Correct answer: Fluorine has one more proton, increasing the pull on the same shell

    Fluorine having one more proton is the correct answer. The extra proton raises the effective nuclear charge acting on electrons in the same principal shell, drawing them inward and making fluorine's radius smaller than that of oxygen.

18. A photoelectron spectrum peak for the 2p electrons of an atom appears at a lower binding energy than the 2s peak of the same atom. What does this indicate?
    • The 2p electrons are held less tightly than the 2s electrons
    • The 2p electrons have more protons
    • The 2s subshell is empty
    • The 2p electrons are core electrons

    Correct answer: The 2p electrons are held less tightly than the 2s electrons

    The 2p electrons being held less tightly is the correct answer. Within the same shell, 2s electrons penetrate closer to the nucleus and feel a higher effective nuclear charge, so they have a higher binding energy than the less penetrating 2p electrons.

19. Which of the following correctly orders the species sodium ion, neon, and fluoride ion from smallest to largest radius, given they are isoelectronic?
    • Fluoride ion, neon, sodium ion
    • Sodium ion, neon, fluoride ion
    • Neon, fluoride ion, sodium ion
    • All three have the same radius

    Correct answer: Sodium ion, neon, fluoride ion

    Sodium ion, neon, fluoride ion is the correct answer. All three have ten electrons, so the species with the most protons, the sodium ion, pulls them in most tightly and is smallest, while the fluoride ion with the fewest protons is largest.

20. What is the primary reason that electron affinity tends to become more negative across a period from left to right?
    • Atoms gain electrons more readily as effective nuclear charge increases
    • Atomic radius increases across a period
    • Shielding increases across a period
    • The number of energy levels increases

    Correct answer: Atoms gain electrons more readily as effective nuclear charge increases

    Atoms gaining electrons more readily as effective nuclear charge increases is the correct answer. A higher effective nuclear charge across a period more strongly attracts an added electron, releasing more energy and making the electron affinity more negative.

21. In the Bohr model of the atom, what happens when an electron absorbs a photon of exactly the right energy?
    • The electron is destroyed
    • The electron jumps to a higher allowed energy level
    • The nucleus gains a proton
    • The atom loses a neutron

    Correct answer: The electron jumps to a higher allowed energy level

    The electron jumps to a higher allowed energy level is the correct answer. Absorbing a photon whose energy matches the gap between levels excites the electron from a lower to a higher quantized orbit, consistent with discrete atomic energy levels.

22. A 3.0-gram sample of an element is found to contain about $6.02 \times 10^{22}$ atoms. If one mole of the element contains $6.02 \times 10^{23}$ atoms, what is the molar mass of the element?
    • 3 grams per mole
    • 30 grams per mole
    • 0.3 grams per mole
    • 300 grams per mole

    Correct answer: 30 grams per mole

    30 grams per mole is the correct answer. The sample contains one tenth of a mole because $6.02 \times 10^{22}$ is one tenth of Avogadro's number, so dividing 3.0 grams by 0.1 mole gives a molar mass of 30 grams per mole.

23. As you compare a sodium chloride lattice to a magnesium oxide lattice, the magnesium oxide melts at a much higher temperature. Which factor in Coulomb's law best explains this difference in lattice strength?
    • Magnesium oxide ions carry larger charges than sodium chloride ions
    • Magnesium oxide ions are much larger than sodium chloride ions
    • Magnesium oxide is a molecular rather than ionic compound
    • Magnesium oxide has weaker electrostatic attractions

    Correct answer: Magnesium oxide ions carry larger charges than sodium chloride ions

    Larger ionic charges in magnesium oxide is the correct answer. Coulomb's law shows lattice energy increases with the product of the ionic charges, and the doubly charged $\ce{Mg^2+}$ and $\ce{O^2-}$ ions attract far more strongly than the singly charged $\ce{Na+}$ and $\ce{Cl-}$ ions, giving a higher melting point.

24. A bonding model describes the percent ionic character of a bond as increasing with the electronegativity difference between two atoms. A bond between two atoms with a large electronegativity difference is best classified as which type?
    • Nonpolar covalent
    • Polar covalent or ionic
    • Metallic
    • Purely covalent with equal sharing

    Correct answer: Polar covalent or ionic

    Polar covalent or ionic is the correct answer. As the electronegativity difference grows, electron sharing becomes increasingly unequal, shifting the bond from polar covalent toward ionic, where electrons are essentially transferred.

25. A central sulfur atom in sulfur tetrafluoride has four bonding pairs and one lone pair. What is its molecular geometry?
    • Tetrahedral
    • Trigonal bipyramidal
    • Seesaw
    • Square planar

    Correct answer: Seesaw

    Seesaw is the correct answer. Five electron domains give a trigonal bipyramidal electron arrangement, and placing the single lone pair in an equatorial position leaves the four bonded atoms in a seesaw shape.

26. A central xenon atom in xenon tetrafluoride is surrounded by four bonding pairs and two lone pairs. What molecular geometry results?
    • Tetrahedral
    • Octahedral
    • Seesaw
    • Square planar

    Correct answer: Square planar

    Square planar is the correct answer. Six electron domains form an octahedral arrangement, and placing the two lone pairs on opposite sides leaves the four bonded fluorine atoms in one plane, a square planar shape.

27. In ethyne (acetylene), each carbon is bonded to one hydrogen and triple-bonded to the other carbon. What hybridization describes each carbon atom?
    • Sp hybridized
    • Sp2 hybridized
    • Sp3 hybridized
    • Unhybridized

    Correct answer: Sp hybridized

    sp hybridized is the correct answer. Each carbon forms two sigma bonds with a linear arrangement, requiring two sp hybrid orbitals, while the two remaining unhybridized p orbitals form the two pi bonds of the triple bond.

28. A triple bond consists of how many sigma bonds and how many pi bonds?
    • Three sigma bonds and zero pi bonds
    • Two sigma bonds and one pi bond
    • Zero sigma bonds and three pi bonds
    • One sigma bond and two pi bonds

    Correct answer: One sigma bond and two pi bonds

    One sigma bond and two pi bonds is the correct answer. A triple bond always contains a single sigma bond from head-on orbital overlap plus two pi bonds formed by the side-to-side overlap of two pairs of unhybridized p orbitals.

29. The nitrate ion can be drawn with the negative charge and double bond placed on different oxygen atoms. Experiments show all three nitrogen-oxygen bonds are identical in length. How does resonance explain this observation?
    • The molecule rapidly oscillates so the bonds appear equal on average
    • Two bonds are single and one is double, but they look similar
    • Electron delocalization spreads the bonding evenly, making all bonds equivalent
    • The structure with the most double bonds is the only real one

    Correct answer: Electron delocalization spreads the bonding evenly, making all bonds equivalent

    Electron delocalization making all bonds equivalent is the correct answer. The pi electrons are spread across all three nitrogen-oxygen bonds rather than fixed in one location, so each bond is identical, intermediate between a single and double bond.

30. Boron trifluoride has a central boron atom bonded to three fluorine atoms with only six electrons around boron. This molecule is best described as which kind of exception to common bonding rules?
    • An expanded octet molecule
    • A molecule with delocalized metallic bonding
    • An electron-deficient molecule with an incomplete octet
    • An ionic compound

    Correct answer: An electron-deficient molecule with an incomplete octet

    An electron-deficient molecule with an incomplete octet is the correct answer. Boron forms three bonds and remains surrounded by only six valence electrons, falling short of an octet, which makes boron trifluoride a stable exception that acts as an electron-pair acceptor.

31. Phosphorus pentachloride has a central phosphorus atom bonded to five chlorine atoms. This molecule illustrates which exception to the octet rule?
    • An expanded octet with more than eight electrons
    • An incomplete octet
    • An odd-electron radical
    • A purely ionic structure

    Correct answer: An expanded octet with more than eight electrons

    An expanded octet is the correct answer. Phosphorus, a period 3 element, can accommodate ten electrons in its valence shell by using available d-orbital space, allowing it to form five bonds in phosphorus pentachloride.

32. The ideal bond angle around a carbon atom with four single bonds and no lone pairs is closest to which value?
    • 90 degrees
    • 109.5 degrees
    • 120 degrees
    • 180 degrees

    Correct answer: 109.5 degrees

    109.5 degrees is the correct answer. Four bonding domains around a central atom adopt a tetrahedral arrangement that maximizes separation, producing the characteristic tetrahedral bond angle of approximately 109.5 degrees.

33. Lone pairs occupy more space than bonding pairs around a central atom. As a result, the H-N-H bond angle in ammonia compared with the ideal tetrahedral angle is best described as which of the following?
    • Larger than 109.5 degrees
    • Slightly smaller than 109.5 degrees
    • Exactly 90 degrees
    • Exactly 180 degrees

    Correct answer: Slightly smaller than 109.5 degrees

    Slightly smaller than 109.5 degrees is the correct answer. The lone pair on nitrogen repels the bonding pairs more strongly than they repel each other, compressing the H-N-H angle to about 107 degrees, just below the ideal tetrahedral value.

34. A compound made of a metal and a nonmetal typically forms which kind of bond, and what does this predict about its electrical conductivity?
    • A covalent bond that conducts only as a gas
    • A metallic bond that conducts in every state
    • A network bond that never conducts
    • An ionic bond that conducts when molten or dissolved but not as a solid

    Correct answer: An ionic bond that conducts when molten or dissolved but not as a solid

    An ionic bond that conducts when molten or dissolved is the correct answer. Metal-nonmetal combinations transfer electrons to form ions, and these ions are locked in place in the solid but become mobile and conductive once the compound is melted or dissolved in water.

35. A carbon-carbon single bond, double bond, and triple bond are compared. Which lists them in order of increasing bond energy (weakest to strongest)?
    • Single, double, triple
    • Triple, double, single
    • Double, single, triple
    • All three bonds have equal energy

    Correct answer: Single, double, triple

    Single, double, triple is the correct answer. As bond order increases, more shared electron pairs hold the nuclei together more strongly, so bond energy rises from the single bond to the double bond to the strongest triple bond.

36. A molecule of carbon tetrachloride contains four polar carbon-chlorine bonds yet is nonpolar overall. Compared with chloroform, which has three carbon-chlorine bonds and one carbon-hydrogen bond, why is chloroform polar?
    • Chloroform contains nonpolar bonds only
    • Carbon tetrachloride has more lone pairs
    • Chloroform's bond dipoles do not cancel because of its less symmetric arrangement
    • Both molecules are equally nonpolar

    Correct answer: Chloroform's bond dipoles do not cancel because of its less symmetric arrangement

    Chloroform's bond dipoles not canceling is the correct answer. In carbon tetrachloride the four identical bond dipoles point symmetrically and cancel, but replacing one chlorine with hydrogen in chloroform breaks that symmetry so the dipoles no longer cancel, leaving a net dipole.

37. A photovoltaic device relies on a semiconductor whose conductivity falls between that of a conductor and an insulator. According to band theory, what distinguishes a semiconductor from an insulator?
    • A semiconductor has no band gap at all
    • A semiconductor has a small band gap that electrons can cross with modest energy
    • A semiconductor has a larger band gap than an insulator
    • A semiconductor conducts only as a liquid

    Correct answer: A semiconductor has a small band gap that electrons can cross with modest energy

    A small band gap electrons can cross is the correct answer. In a semiconductor the energy gap between the filled valence band and empty conduction band is small enough that added energy promotes electrons into the conduction band, unlike an insulator's large gap.

38. Two compounds share the molecular formula $\ce{C2H6O}$ but differ in their structures and properties, one being an alcohol and the other an ether. What is the relationship between these two compounds?
    • They are isotopes
    • They are allotropes
    • They are structural isomers
    • They are resonance structures

    Correct answer: They are structural isomers

    They are structural isomers is the correct answer. Compounds with the same molecular formula but different arrangements of their atoms are structural isomers, which is why the same formula $\ce{C2H6O}$ can describe both an alcohol and an ether with distinct properties.

39. Which intermolecular force is responsible for the unusually high boiling point of water relative to other molecules of similar size?
    • London dispersion forces
    • Ion-dipole forces
    • Metallic bonding
    • Hydrogen bonding

    Correct answer: Hydrogen bonding

    Hydrogen bonding is the correct answer. Water's O-H bonds let each molecule form strong hydrogen bonds with neighbors, which requires extra energy to break and raises the boiling point well above that of similarly sized molecules lacking such bonds.

40. Hydrogen bonding can occur when hydrogen is covalently bonded to which set of atoms?
    • Carbon, silicon, or sulfur
    • Chlorine, bromine, or iodine
    • Sodium, potassium, or lithium
    • Fluorine, oxygen, or nitrogen

    Correct answer: Fluorine, oxygen, or nitrogen

    Fluorine, oxygen, or nitrogen is the correct answer. Hydrogen bonding requires hydrogen attached to one of these small, highly electronegative atoms, which creates the strong partial charges needed for the interaction.

41. Which intermolecular force arises between two polar molecules that each possess a permanent dipole?
    • London dispersion only
    • Covalent bonding
    • Dipole-dipole forces
    • Nuclear forces

    Correct answer: Dipole-dipole forces

    Dipole-dipole forces is the correct answer. Polar molecules have permanent dipoles, so the positive end of one molecule attracts the negative end of another, producing dipole-dipole attractions between them.

42. London dispersion forces originate from which momentary condition within molecules?
    • Permanent ionic charges
    • Fixed covalent bonds breaking
    • Nuclear spin alignment
    • Temporary, instantaneous dipoles from shifting electron clouds

    Correct answer: Temporary, instantaneous dipoles from shifting electron clouds

    Temporary instantaneous dipoles is the correct answer. London dispersion forces come from the constant random movement of electrons, which momentarily creates uneven charge distributions that induce dipoles in neighboring molecules.

43. Among molecules of similar shape, which factor most increases the strength of London dispersion forces?
    • A smaller number of electrons
    • Higher ionic charge
    • Stronger covalent bonds within the molecule
    • A larger, more polarizable electron cloud

    Correct answer: A larger, more polarizable electron cloud

    A larger, more polarizable electron cloud is the correct answer. More electrons spread over a larger volume are easier to distort, producing stronger temporary dipoles and therefore stronger London dispersion forces.

44. An ion-dipole interaction is the principal force in which situation?
    • Two nonpolar gas molecules colliding
    • Two oxygen molecules in liquid oxygen
    • A pair of methane molecules
    • A sodium ion surrounded by water molecules in solution

    Correct answer: A sodium ion surrounded by water molecules in solution

    A sodium ion surrounded by water molecules is the correct answer. Ion-dipole forces occur when an ion attracts the oppositely charged end of a polar molecule, as happens when water hydrates dissolved ions.

45. Why does a branched isomer of a hydrocarbon generally boil at a lower temperature than its straight-chain isomer of the same formula?
    • Branched molecules are more polar
    • Branched molecules contain hydrogen bonds
    • Branched molecules have a smaller surface area for contact, weakening dispersion forces
    • Branched molecules have more electrons

    Correct answer: Branched molecules have a smaller surface area for contact, weakening dispersion forces

    Smaller surface area for contact is the correct answer. A more compact, branched shape reduces the surface contact between molecules, weakening London dispersion forces and lowering the boiling point compared with an extended straight chain.

46. As intermolecular forces in a liquid become stronger, what happens to the liquid's vapor pressure at a given temperature?
    • Vapor pressure increases
    • Vapor pressure is unaffected
    • Vapor pressure becomes infinite
    • Vapor pressure decreases

    Correct answer: Vapor pressure decreases

    Vapor pressure decreases is the correct answer. Stronger intermolecular forces hold molecules in the liquid more tightly, so fewer escape into the vapor phase, lowering the measured vapor pressure.

47. A liquid with a high vapor pressure at room temperature is best described as which of the following?
    • Viscous
    • Volatile
    • Ionic
    • Crystalline

    Correct answer: Volatile

    Volatile is the correct answer. A volatile liquid evaporates readily because its weak intermolecular forces allow many molecules to enter the vapor phase, giving it a high vapor pressure.

48. A liquid boils when its vapor pressure becomes equal to which quantity?
    • The triple point pressure
    • Zero
    • The external atmospheric pressure
    • The critical pressure

    Correct answer: The external atmospheric pressure

    The external atmospheric pressure is the correct answer. Boiling occurs throughout a liquid when its vapor pressure rises to match the surrounding pressure, allowing bubbles of vapor to form within the liquid.

49. Why does water boil at a lower temperature at high altitude than at sea level?
    • The water is colder at altitude
    • The water becomes more polar
    • The atmospheric pressure is lower, so vapor pressure matches it sooner
    • Hydrogen bonds disappear at altitude

    Correct answer: The atmospheric pressure is lower, so vapor pressure matches it sooner

    Lower atmospheric pressure is the correct answer. At high altitude the reduced air pressure means the liquid's vapor pressure equals the surroundings at a lower temperature, so boiling occurs below 100 degrees Celsius.

50. Which property of a liquid measures its resistance to flow and increases with stronger intermolecular forces?
    • Density
    • Solubility
    • Vapor pressure
    • Viscosity

    Correct answer: Viscosity

    Viscosity is the correct answer. Viscosity reflects internal resistance to flow, and liquids with stronger intermolecular attractions, such as honey, have higher viscosity than those with weak forces.

51. Which type of solid is held together by a continuous network of covalent bonds, giving it a very high melting point?
    • Molecular solid
    • Ionic solid
    • Covalent network solid
    • Metallic solid

    Correct answer: Covalent network solid

    Covalent network solid is the correct answer. In a covalent network solid such as diamond, every atom is bonded to its neighbors by strong covalent bonds throughout the crystal, requiring enormous energy to melt.

52. Why are metals good conductors of electricity in the solid state?
    • They are made of separate molecules
    • They contain delocalized electrons that move freely
    • They contain mobile negative ions
    • They have covalent network structures

    Correct answer: They contain delocalized electrons that move freely

    Delocalized electrons that move freely is the correct answer. Metallic bonding produces a sea of delocalized valence electrons that can drift through the lattice, allowing metals to conduct electricity even as solids.

53. Molecular solids, such as solid carbon dioxide, generally have low melting points because they are held together by what?
    • Strong ionic bonds
    • Metallic bonds
    • A covalent network
    • Relatively weak intermolecular forces between discrete molecules

    Correct answer: Relatively weak intermolecular forces between discrete molecules

    Relatively weak intermolecular forces is the correct answer. Molecular solids consist of individual molecules held together only by intermolecular attractions, which are far weaker than the bonds in ionic or covalent network solids, so they melt easily.

54. Why does solid sodium chloride not conduct electricity, while molten sodium chloride does?
    • Solid salt has no ions
    • In the solid, ions are locked in place, but melting frees them to move
    • The molten form loses its charge
    • Heating destroys the ionic bonds permanently

    Correct answer: In the solid, ions are locked in place, but melting frees them to move

    Ions locked in place until melting is the correct answer. Conduction requires mobile charge carriers; in solid sodium chloride the ions are fixed in the lattice, but melting allows the ions to move and carry current.

55. At constant temperature and amount, doubling the pressure on an ideal gas changes its volume to what fraction of the original?
    • One half
    • Twice
    • Four times
    • Unchanged

    Correct answer: One half

    One half is the correct answer. Boyle's law makes pressure and volume inversely proportional at constant temperature, so doubling the pressure reduces the volume to half its original value.

56. In the ideal gas law, what does the term n represent?
    • The number of moles of gas
    • The pressure of the gas
    • The temperature in Celsius
    • The number of collisions

    Correct answer: The number of moles of gas

    The number of moles of gas is the correct answer. In the ideal gas law relating pressure, volume, temperature, and amount, the symbol n stands for the quantity of gas measured in moles.

57. For the ideal gas law to give correct results, the temperature must be expressed in which unit?
    • Degrees Celsius
    • Degrees Fahrenheit
    • Kelvin
    • Joules

    Correct answer: Kelvin

    Kelvin is the correct answer. Gas law calculations require absolute temperature in kelvin because the relationships are based on a scale that starts at absolute zero, where particle motion is at a minimum.

58. If a sealed rigid container of gas is cooled, what happens to the pressure inside, assuming the amount of gas stays constant?
    • The pressure increases
    • The pressure remains constant
    • The pressure decreases
    • The pressure becomes negative

    Correct answer: The pressure decreases

    The pressure decreases is the correct answer. At constant volume, pressure is directly proportional to absolute temperature, so cooling the gas reduces the kinetic energy of its particles and lowers the pressure.

59. According to kinetic molecular theory, collisions between ideal gas particles are described as what?
    • Perfectly elastic, conserving total kinetic energy
    • Inelastic, losing energy each time
    • Always sticking together
    • Causing chemical reactions

    Correct answer: Perfectly elastic, conserving total kinetic energy

    Perfectly elastic is the correct answer. Kinetic molecular theory assumes gas particle collisions are perfectly elastic, meaning no kinetic energy is lost when particles collide with each other or the container walls.

60. What causes the pressure that a gas exerts on the walls of its container?
    • Gravity pulling on the gas
    • Chemical bonds forming with the walls
    • The color of the gas
    • Collisions of gas particles with the container walls

    Correct answer: Collisions of gas particles with the container walls

    Collisions with the container walls is the correct answer. Gas pressure results from the countless impacts of fast-moving particles striking the container surfaces, with more frequent or forceful collisions producing higher pressure.

61. At a given temperature, two gases of different molar mass have what in common according to kinetic molecular theory?
    • The same average speed
    • The same average kinetic energy
    • The same mass per particle
    • The same volume per particle

    Correct answer: The same average kinetic energy

    The same average kinetic energy is the correct answer. Average kinetic energy depends only on temperature, so two gases at the same temperature share the same average kinetic energy even though their speeds differ with molar mass.

62. When a gas is collected over water, the measured total pressure includes the partial pressure of which additional substance?
    • Water vapor
    • Dissolved salt
    • Oxygen only
    • Carbon dioxide always

    Correct answer: Water vapor

    Water vapor is the correct answer. A gas collected over water becomes saturated with water vapor, so by Dalton's law the total pressure is the sum of the dry gas pressure and the water vapor pressure.

63. A real gas shows the greatest deviation from ideal behavior when its particles experience which conditions?
    • High temperature where motion overcomes attractions
    • Very low pressure
    • Complete vacuum
    • Low temperature where attractions become significant

    Correct answer: Low temperature where attractions become significant

    Low temperature where attractions become significant is the correct answer. At low temperature the slower particles are more affected by intermolecular attractions, causing the gas to deviate noticeably from ideal behavior.

64. Which gas would be expected to behave most ideally under ordinary conditions?
    • Water vapor, with strong hydrogen bonding
    • Ammonia, a polar molecule
    • Helium, a small nonpolar atom with weak forces
    • Hydrogen fluoride, which hydrogen bonds

    Correct answer: Helium, a small nonpolar atom with weak forces

    Helium is the correct answer. Helium atoms are tiny and experience only very weak intermolecular forces, so they closely approximate the ideal gas assumptions of negligible volume and no attractions.

65. At standard conditions, how does the molar mass of a gas relate to its density?
    • Higher molar mass gives higher gas density
    • Higher molar mass gives lower density
    • Molar mass has no effect on density
    • Density depends only on color

    Correct answer: Higher molar mass gives higher gas density

    Higher molar mass gives higher density is the correct answer. At the same temperature and pressure equal volumes hold equal numbers of molecules, so a gas with heavier molecules packs more mass into that volume and is denser.

66. Mass percent of a solute in a solution is calculated as the mass of solute divided by which quantity, times 100?
    • The volume of solvent
    • The total mass of the solution
    • The number of moles of solute
    • The density of the solvent

    Correct answer: The total mass of the solution

    The total mass of the solution is the correct answer. Mass percent expresses concentration as the mass of solute divided by the total mass of the solution, then multiplied by 100 to give a percentage.

67. Parts per million is a useful concentration unit primarily for solutions that are what?
    • Very concentrated
    • Extremely dilute
    • Pure solvents
    • Saturated

    Correct answer: Extremely dilute

    Extremely dilute is the correct answer. Parts per million expresses very small amounts of solute relative to the solution, making it convenient for trace concentrations such as pollutants in water.

68. When an ionic compound dissolves in water and breaks apart into ions, the resulting solution can do what?
    • Conduct electricity as an electrolyte
    • Stop all molecular motion
    • Become a pure substance
    • Lose all its mass

    Correct answer: Conduct electricity as an electrolyte

    Conduct electricity as an electrolyte is the correct answer. The mobile ions produced when an ionic compound dissociates allow the solution to carry electric current, classifying it as an electrolyte.

69. A sugar solution does not conduct electricity well because dissolved sugar does what?
    • Forms many free ions
    • Remains as neutral molecules without producing ions
    • Breaks into protons and neutrons
    • Reacts with the electrodes

    Correct answer: Remains as neutral molecules without producing ions

    Remains as neutral molecules is the correct answer. Sugar is a molecular compound that dissolves without dissociating into ions, so its solution lacks the mobile charges needed to conduct electricity and is a nonelectrolyte.

70. In the dissolving process, energy is required to do which of the following?
    • Separate solute particles and pull solvent molecules apart
    • Create new solute particles
    • Increase the solute's molar mass
    • Form covalent bonds within the solvent

    Correct answer: Separate solute particles and pull solvent molecules apart

    Separate solute and solvent particles is the correct answer. Dissolving requires energy to overcome the attractions among solute particles and among solvent particles before new solute-solvent interactions can form.

71. A supersaturated solution contains how much solute relative to a normally saturated solution at the same temperature?
    • Less solute
    • The same amount of solute
    • More dissolved solute than normally possible
    • No solute

    Correct answer: More dissolved solute than normally possible

    More dissolved solute than normally possible is the correct answer. A supersaturated solution holds more dissolved solute than its saturation point allows, an unstable condition usually formed by carefully cooling a hot saturated solution.

72. Adding a small seed crystal to a supersaturated solution typically causes what to happen?
    • The excess solute rapidly crystallizes out
    • The solution boils
    • All solute dissolves further
    • The solvent evaporates

    Correct answer: The excess solute rapidly crystallizes out

    The excess solute rapidly crystallizes out is the correct answer. A seed crystal gives the unstable excess solute a surface to deposit on, triggering rapid crystallization until the solution returns to saturation.

73. Which separation technique would best recover dissolved salt from saltwater?
    • Filtration through paper
    • Evaporation of the water to leave the solid salt
    • Decanting the top layer
    • Using a magnet

    Correct answer: Evaporation of the water to leave the solid salt

    Evaporation of the water is the correct answer. Because the salt is dissolved and passes through filters, removing the water by evaporation leaves the solid salt behind, accomplishing the separation.

74. In paper chromatography, the retention factor is calculated by dividing the distance traveled by the component by which quantity?
    • The distance traveled by the solvent front
    • The width of the paper
    • The mass of the sample
    • The time of the experiment

    Correct answer: The distance traveled by the solvent front

    The distance traveled by the solvent front is the correct answer. The retention factor compares how far a component moved to how far the solvent front advanced, giving a value between zero and one that characterizes the component.

75. A component with a high retention factor in chromatography has which characteristic?
    • Strong attraction to the stationary phase
    • Weak interaction with the mobile phase
    • It traveled a large fraction of the solvent distance
    • It did not move at all

    Correct answer: It traveled a large fraction of the solvent distance

    It traveled a large fraction of the solvent distance is the correct answer. A high retention factor means the component moved nearly as far as the solvent front, indicating it interacts more with the mobile phase than the stationary phase.

76. In the photoelectric effect, electrons are ejected from a metal surface only when the incoming light has a frequency above what value?
    • The threshold frequency
    • Any frequency at all
    • Zero frequency
    • The boiling frequency

    Correct answer: The threshold frequency

    The threshold frequency is the correct answer. The photoelectric effect requires each photon to carry enough energy to free an electron, so light below the metal's threshold frequency ejects no electrons regardless of its intensity.

77. In the photoelectric effect, increasing the intensity of light above the threshold frequency increases what?
    • The energy of each ejected electron
    • The number of electrons ejected per second
    • The threshold frequency
    • The wavelength of the light

    Correct answer: The number of electrons ejected per second

    The number of electrons ejected per second is the correct answer. Above the threshold, brighter light delivers more photons, ejecting more electrons, although the kinetic energy of each electron depends on frequency, not intensity.

78. A calibration curve in a Beer-Lambert experiment is typically constructed by plotting absorbance against which variable?
    • The temperature of each sample
    • Known concentrations of standard solutions
    • The mass of the cuvette
    • The time of measurement

    Correct answer: Known concentrations of standard solutions

    Known concentrations of standard solutions is the correct answer. Plotting measured absorbance versus known concentrations produces a straight calibration line that lets an unknown sample's concentration be read from its absorbance.

79. Which intermolecular force is responsible for the attraction between two molecules that each have a permanent dipole?
    • Dipole-dipole forces
    • London dispersion forces only
    • Covalent bonding
    • Ionic bonding

    Correct answer: Dipole-dipole forces

    Dipole-dipole forces is the correct answer. When molecules possess permanent dipoles, the positive end of one molecule attracts the negative end of a neighboring molecule, creating dipole-dipole attractions.

80. Hydrogen bonding occurs when hydrogen is bonded directly to which group of highly electronegative atoms?
    • Carbon, silicon, or boron
    • Fluorine, oxygen, or nitrogen
    • Sodium, potassium, or lithium
    • Chlorine, bromine, or iodine

    Correct answer: Fluorine, oxygen, or nitrogen

    Fluorine, oxygen, or nitrogen is the correct answer. Hydrogen bonding is an especially strong dipole interaction that requires hydrogen to be attached to one of these small, highly electronegative atoms.

81. The unusually strong attraction between a dissolved ion and the polar water molecules surrounding it is best classified as which interaction?
    • London dispersion force
    • Hydrogen bond
    • Ion-dipole force
    • Metallic bond

    Correct answer: Ion-dipole force

    Ion-dipole force is the correct answer. An ion-dipole interaction forms between a charged ion and the partial charges of a polar molecule such as water, and it is central to dissolving ionic compounds.

82. Why does a larger, more easily distorted electron cloud tend to produce stronger London dispersion forces?
    • Higher polarizability allows larger temporary dipoles to form
    • It reduces the molar mass of the molecule
    • It eliminates all permanent dipoles
    • It increases the number of covalent bonds

    Correct answer: Higher polarizability allows larger temporary dipoles to form

    Higher polarizability is the correct answer. A larger, more polarizable electron cloud can shift more readily, producing larger instantaneous dipoles and therefore stronger London dispersion attractions.

83. Diamond is extremely hard and has a very high melting point because it is classified as which kind of solid?
    • Molecular solid
    • Metallic solid
    • Ionic solid
    • Covalent network solid

    Correct answer: Covalent network solid

    Covalent network solid is the correct answer. In a covalent network solid like diamond, every atom is joined to its neighbors by strong covalent bonds extending throughout the structure, requiring great energy to break apart.

84. At room temperature, a molecular solid such as solid carbon dioxide tends to have a low melting point because its particles are held together by what?
    • Relatively weak intermolecular forces
    • Strong covalent network bonds
    • Ionic lattice attractions
    • Delocalized metallic electrons

    Correct answer: Relatively weak intermolecular forces

    Relatively weak intermolecular forces is the correct answer. Molecular solids consist of discrete molecules held in place only by intermolecular attractions, which are far weaker than covalent or ionic bonds, so they melt at low temperatures.

85. Among liquids of similar size, a liquid with stronger intermolecular forces is expected to have which property?
    • Higher vapor pressure at a given temperature
    • Lower viscosity
    • Lower vapor pressure at a given temperature
    • No surface tension

    Correct answer: Lower vapor pressure at a given temperature

    Lower vapor pressure is the correct answer. Strong intermolecular forces hold molecules in the liquid more tightly, so fewer molecules escape into the vapor and the equilibrium vapor pressure is lower.

86. Honey flows more slowly than water largely because honey has a higher value of which property?
    • Vapor pressure
    • Viscosity
    • Effusion rate
    • Mole fraction

    Correct answer: Viscosity

    Viscosity is the correct answer. Viscosity measures a liquid's resistance to flow, and stronger intermolecular attractions among honey's molecules give it a high viscosity compared with water.

87. The concentration unit expressing moles of solute per kilogram of solvent is known as what?
    • Molarity
    • Molality
    • Mole fraction
    • Mass percent

    Correct answer: Molality

    Molality is the correct answer. Molality is defined as moles of solute divided by kilograms of solvent, a unit that does not change with temperature because it relies on mass rather than volume.

88. A very dilute concentration of a contaminant is often reported in parts per million, which is most nearly equivalent to which ratio?
    • Milligrams of solute per kilogram of solution
    • Moles of solute per liter of solution
    • Grams of solute per gram of solute
    • Liters of gas per mole of gas

    Correct answer: Milligrams of solute per kilogram of solution

    Milligrams of solute per kilogram of solution is the correct answer. One part per million corresponds to one milligram of solute in one kilogram of solution, a convenient way to express extremely small concentrations.

89. The mass percent of a solute in a solution is calculated by dividing the mass of the solute by which quantity and multiplying by 100?
    • The volume of the solvent
    • The total mass of the solution
    • The number of moles of solvent
    • The density of the solute

    Correct answer: The total mass of the solution

    The total mass of the solution is the correct answer. Mass percent equals the solute mass divided by the total solution mass, times 100, expressing composition as a percentage by mass.

90. The root-mean-square speed of gas molecules at a fixed temperature depends on molar mass in what way?
    • It increases as molar mass increases
    • It is independent of molar mass
    • It decreases as molar mass increases
    • It equals the molar mass directly

    Correct answer: It decreases as molar mass increases

    It decreases as molar mass increases is the correct answer. Root-mean-square speed is inversely related to $\sqrt{\text{molar mass}}$, so at the same temperature heavier molecules move more slowly than lighter ones.

91. When a sample of zinc reacts with acid to produce hydrogen gas collected over water, the measured total pressure must be corrected by subtracting which quantity to find the pressure of the dry gas?
    • The vapor pressure of water
    • The atmospheric carbon dioxide pressure
    • The molar mass of zinc
    • The volume of the container

    Correct answer: The vapor pressure of water

    The vapor pressure of water is the correct answer. A gas collected over water is mixed with water vapor, so by Dalton's law the water vapor pressure is subtracted from the total to obtain the partial pressure of the dry gas.

92. Glass is often described as an amorphous solid, which means its particles are arranged in what manner?
    • In a highly ordered, repeating crystalline lattice
    • Without long-range order, lacking a regular repeating pattern
    • As freely moving particles like a gas
    • In separate ionic layers

    Correct answer: Without long-range order, lacking a regular repeating pattern

    Without long-range order is the correct answer. An amorphous solid such as glass has particles fixed in place but lacking the regular, repeating arrangement found in crystalline solids.

93. Two aqueous solutions are mixed and no solid forms, no gas escapes, and no water is produced. What does this observation most strongly suggest about the mixed ions?
    • A precipitation reaction occurred
    • No net reaction took place and all ions are spectators
    • A redox reaction occurred
    • A combustion reaction occurred

    Correct answer: No net reaction took place and all ions are spectators

    No net reaction took place is the correct answer. When mixing two solutions yields no precipitate, gas, or water, all ions remain dissolved and unchanged as spectators, so there is no net ionic reaction.

94. When writing a complete ionic equation, which type of substance is split into separate ions?
    • Solid precipitates
    • Pure liquids such as water
    • Soluble strong electrolytes in aqueous solution
    • Gases released from solution

    Correct answer: Soluble strong electrolytes in aqueous solution

    Soluble strong electrolytes in aqueous solution is the correct answer. In a complete ionic equation, soluble strong electrolytes are written as dissociated ions, while solids, liquids, and gases are kept in their molecular form.

95. What is the oxidation number of sulfur in the sulfate ion, which carries a 2 minus charge with four oxygen atoms each at minus 2?
    • Plus 2
    • Plus 4
    • Minus 6
    • Plus 6

    Correct answer: Plus 6

    Plus 6 is the correct answer. With four oxygens contributing minus 8 total and the overall ion charge being minus 2, sulfur must be plus 6 so the oxidation numbers sum to the ion charge.

96. In the reaction where magnesium metal reacts with oxygen gas to form magnesium oxide, what happens to the magnesium?
    • It is oxidized as it loses electrons
    • It is reduced as it gains electrons
    • It acts as a spectator ion
    • It is neither oxidized nor reduced

    Correct answer: It is oxidized as it loses electrons

    It is oxidized as it loses electrons is the correct answer. Magnesium goes from an oxidation state of zero to plus 2 by losing electrons, which is the definition of being oxidized.

97. A student combines lead nitrate solution with potassium iodide solution and observes a bright yellow solid. Which classification best fits this reaction?
    • Combustion
    • Double-replacement precipitation
    • Single-replacement
    • Acid-base neutralization

    Correct answer: Double-replacement precipitation

    Double-replacement precipitation is the correct answer. The cations and anions exchange partners to form lead iodide, an insoluble yellow solid that precipitates, which is a double-replacement precipitation reaction.

98. Based on the balanced equation $\ce{N2 + 3H2 -> 2NH3}$, how many moles of ammonia form from 6 moles of hydrogen gas, assuming nitrogen is in excess?
    • 2 moles
    • 6 moles
    • 4 moles
    • 3 moles

    Correct answer: 4 moles

    4 moles is the correct answer. The mole ratio of hydrogen to ammonia is 3 to 2, so 6 moles of hydrogen produces 4 moles of ammonia.

99. To balance the equation $\ce{Fe + O2 -> Fe2O3}$, what coefficient must be placed in front of iron?
    • 2
    • 3
    • 6
    • 4

    Correct answer: 4

    4 is the correct answer. The balanced equation is $\ce{4Fe + 3O2 -> 2Fe2O3}$, which provides equal numbers of iron and oxygen atoms on both sides.

100. In a titration, a chemist slowly adds base to an acid until the reaction is just complete. What is the main purpose of recording the exact volume of base delivered?
     • To calculate the moles of base used to reach the equivalence point
     • To measure the temperature of the solution
     • To determine the color of the indicator
     • To find the density of the acid

     Correct answer: To calculate the moles of base used to reach the equivalence point

     To calculate the moles of base used is the correct answer. The measured volume of titrant, combined with its known concentration, gives the moles of base needed to reach the equivalence point and thus the moles of acid.

101. If 30.0 milliliters of sodium hydroxide is required to neutralize an acid, and the base concentration is 0.200 molar, how many moles of hydroxide were delivered?
     • 0.0600 mole
     • 0.00600 mole
     • 0.0300 mole
     • 0.200 mole

     Correct answer: 0.00600 mole

     0.00600 mole is the correct answer. Multiplying the volume of 0.0300 liter by the concentration of 0.200 molar gives 0.00600 mole of hydroxide ion.

102. Which general pattern describes a decomposition reaction?
     • Two elements combine to form one compound
     • One compound breaks down into two or more simpler substances
     • A more reactive element displaces another from a compound
     • An acid reacts with a base to form salt and water

     Correct answer: One compound breaks down into two or more simpler substances

     One compound breaks down into simpler substances is the correct answer. A decomposition reaction takes a single compound and splits it into two or more simpler products, which is the opposite of synthesis.

103. A hydrocarbon undergoes incomplete combustion in limited oxygen. Which product, in addition to water, may form that would not appear in complete combustion?
     • Sodium chloride
     • Sulfuric acid
     • Carbon monoxide
     • Ammonia

     Correct answer: Carbon monoxide

     Carbon monoxide is the correct answer. When oxygen is limited, incomplete combustion can produce carbon monoxide or soot rather than fully oxidizing all carbon to carbon dioxide.

104. A reaction begins with 4 moles of one reactant and 4 moles of a second reactant that combine in a 1 to 2 ratio. Which reactant is limiting?
     • The reactant needed in the 1 ratio, since it determines the amount
     • The reactant needed in the 2 ratio, since only enough is present for 2 of the first
     • Neither, because both amounts are equal
     • Both run out at exactly the same time

     Correct answer: The reactant needed in the 2 ratio, since only enough is present for 2 of the first

     The reactant needed in the 2 ratio is the correct answer. With only 4 moles of the reactant required at twice the rate, just 2 moles of the first reactant can react, so the 2-ratio reactant runs out first and limits the reaction.

105. A reaction has a theoretical yield of 50.0 grams but produces only 40.0 grams of product. What is the percent yield?
     • 125 percent
     • 10.0 percent
     • 90.0 percent
     • 80.0 percent

     Correct answer: 80.0 percent

     80.0 percent is the correct answer. Dividing the actual yield of 40.0 grams by the theoretical yield of 50.0 grams and multiplying by 100 gives a percent yield of 80.0 percent.

106. Why is a primary standard, a highly pure and stable substance of known mass, useful in titration?
     • It allows the precise concentration of a titrant to be established
     • It changes color at the endpoint
     • It speeds up the neutralization reaction
     • It increases the volume of the analyte

     Correct answer: It allows the precise concentration of a titrant to be established

     It allows the precise concentration of a titrant to be established is the correct answer. A primary standard's known purity and mass let a chemist standardize a solution, giving an accurate titrant concentration for later analyses.

107. In the half-reaction in which a copper ion gains two electrons to become copper metal, this half-reaction represents which process?
     • Oxidation
     • Reduction
     • Neutralization
     • Precipitation

     Correct answer: Reduction

     Reduction is the correct answer. Gaining electrons lowers the copper ion's oxidation state from plus 2 to zero, and a gain of electrons is by definition a reduction half-reaction.

108. Which statement correctly describes the role of the oxidizing agent in a redox reaction?
     • It loses electrons and is itself oxidized
     • It speeds up the reaction without changing
     • It gains electrons and is itself reduced
     • It remains a spectator throughout

     Correct answer: It gains electrons and is itself reduced

     It gains electrons and is itself reduced is the correct answer. The oxidizing agent causes another species to be oxidized by accepting its electrons, and in doing so the oxidizing agent is reduced.

109. When the equation for the reaction of aluminum with copper sulfate to form aluminum sulfate and copper is balanced, the aluminum coefficient is 2. What does this single-replacement reaction require to proceed?
     • Aluminum must be more reactive than copper in the activity series
     • Copper must be a gas
     • The solution must be acidic
     • A catalyst must be added

     Correct answer: Aluminum must be more reactive than copper in the activity series

     Aluminum must be more reactive than copper is the correct answer. A single-replacement reaction proceeds only when the free metal is higher in the activity series than the metal ion it displaces, and aluminum outranks copper.

110. How is the theoretical yield of a product determined before a reaction is carried out?
     • By weighing the product after the reaction
     • By measuring the temperature change
     • By multiplying the moles of reactants by their densities
     • By using stoichiometry from the limiting reactant and the balanced equation

     Correct answer: By using stoichiometry from the limiting reactant and the balanced equation

     By using stoichiometry from the limiting reactant is the correct answer. The theoretical yield is calculated from the moles of limiting reactant and the mole ratios in the balanced equation, predicting the maximum possible product.

111. In the reaction $\ce{2KClO3 -> 2KCl + 3O2}$, how many moles of oxygen gas are produced when 4 moles of potassium chlorate decompose?
     • 6 moles
     • 4 moles
     • 3 moles
     • 8 moles

     Correct answer: 6 moles

     6 moles is the correct answer. The mole ratio of potassium chlorate to oxygen is 2 to 3, so 4 moles of potassium chlorate yields 6 moles of oxygen gas.

112. Which observation most clearly indicates that a chemical reaction has produced a gas?
     • The solution becomes more transparent
     • Bubbles form and rise out of the mixture
     • The container becomes heavier
     • The liquid level rises slightly

     Correct answer: Bubbles form and rise out of the mixture

     Bubbles form and rise out of the mixture is the correct answer. The steady release of bubbles that escape the solution is direct evidence that a gaseous product has been generated by the reaction.

113. A chemist wants to identify the spectator ions in the reaction of barium chloride with sodium sulfate, which forms a barium sulfate precipitate. Which ions are the spectators?
     • Barium and sulfate ions
     • Sodium and chloride ions
     • Barium and chloride ions
     • Sulfate and sodium ions

     Correct answer: Sodium and chloride ions

     Sodium and chloride ions is the correct answer. Barium and sulfate combine into the insoluble precipitate, while the sodium and chloride ions remain dissolved and unchanged, making them the spectators.

114. Why does balancing the charge, not just the atoms, matter when writing a balanced net ionic equation?
     • Because charge must be conserved just as atoms are conserved
     • Because charge changes the molar mass of products
     • Because charge determines the color of the precipitate
     • Because charge has no effect and only atoms matter

     Correct answer: Because charge must be conserved just as atoms are conserved

     Because charge must be conserved is the correct answer. A correct net ionic equation balances both the number of atoms and the total electrical charge on each side, since charge, like mass, is conserved in a reaction.

115. The average rate of a reaction over a time interval is calculated as the change in concentration of a species divided by what?
     • The elapsed time interval
     • The temperature of the system
     • The total volume
     • The number of products

     Correct answer: The elapsed time interval

     The elapsed time interval is the correct answer. Average rate equals the change in a reactant or product concentration divided by the change in time over the interval being measured.

116. For the reaction 2A produces B, the rate of disappearance of A compares to the rate of appearance of B in what way?
     • A disappears at the same rate B appears
     • A disappears half as fast as B appears
     • A disappears twice as fast as B appears
     • A disappears four times as fast as B appears

     Correct answer: A disappears twice as fast as B appears

     A disappears twice as fast is the correct answer. Stoichiometry shows two A consumed for each B formed, so the magnitude of A's rate of disappearance is twice the rate B appears.

117. In the rate law rate equals k times the concentration of A, the symbol k represents which quantity?
     • The reaction order
     • The activation energy
     • The equilibrium constant
     • The rate constant

     Correct answer: The rate constant

     The rate constant is the correct answer. The proportionality factor k in a rate law is called the rate constant, and its value depends on temperature and the presence of a catalyst.

118. The integrated rate law for a first-order reaction produces a straight line when which quantity is plotted against time?
     • The natural log of the reactant concentration
     • The reactant concentration itself
     • The inverse of the reactant concentration
     • The square of the concentration

     Correct answer: The natural log of the reactant concentration

     The natural log of the reactant concentration is the correct answer. A first-order reaction gives a linear plot when ln of the concentration is graphed versus time, with a slope of negative k.

119. A linear plot of one over the reactant concentration versus time indicates a reaction of which order?
     • Zero order
     • Second order
     • First order
     • Third order

     Correct answer: Second order

     Second order is the correct answer. A second-order reaction in a single reactant gives a straight line when the inverse of the concentration is plotted against time, with a slope equal to k.

120. A linear plot of reactant concentration versus time, with a negative slope, is characteristic of a reaction of which order?
     • First order
     • Second order
     • Zero order
     • Half order

     Correct answer: Zero order

     Zero order is the correct answer. For a zero-order reaction the concentration decreases linearly with time, so a plot of concentration versus time is a straight line whose slope equals negative k.

121. In an initial-rates experiment, doubling reactant A's concentration leaves the rate unchanged. What is the order with respect to A?
     • Second order
     • First order
     • Third order
     • Zero order

     Correct answer: Zero order

     Zero order is the correct answer. If changing a reactant's concentration has no effect on the rate, the reaction is zero order with respect to that reactant.

122. In an initial-rates experiment, doubling reactant B's concentration causes the rate to increase by a factor of four. What is the order with respect to B?
     • Second order
     • First order
     • Zero order
     • Third order

     Correct answer: Second order

     Second order is the correct answer. When doubling a concentration multiplies the rate by four, which is two squared, the reaction is second order with respect to that reactant.

123. The units of the rate constant for a first-order reaction are which of the following?
     • Molarity per second
     • Inverse seconds
     • Inverse molarity per second
     • Molarity squared per second

     Correct answer: Inverse seconds

     Inverse seconds is the correct answer. Because a first-order rate equals k times a concentration, k must carry units of reciprocal time so the product yields molarity per second.

124. The Arrhenius equation describes how the rate constant depends primarily on which two factors?
     • Pressure and volume
     • Concentration and color
     • Temperature and activation energy
     • Mass and density

     Correct answer: Temperature and activation energy

     Temperature and activation energy is the correct answer. The Arrhenius equation relates the rate constant to the absolute temperature and the activation energy through an exponential term.

125. In the Arrhenius equation, the factor A is given which name?
     • The activation energy
     • The rate order
     • The half-life
     • The frequency factor

     Correct answer: The frequency factor

     The frequency factor is the correct answer. The A term, also called the pre-exponential or frequency factor, reflects the frequency of collisions with correct orientation in the Arrhenius model.

126. A reaction is first order in A and first order in B. What is the overall reaction order?
     • Second order
     • First order
     • Zero order
     • Third order

     Correct answer: Second order

     Second order is the correct answer. The overall order is the sum of the individual orders, so first order in A plus first order in B gives an overall order of two.

127. A reaction mechanism has a slow first step. The overall rate law should be based on which step?
     • The fast step
     • The slow first step
     • The final step always
     • The average of all steps

     Correct answer: The slow first step

     The slow first step is the correct answer. When the first step is rate-determining, the overall rate law is written from that slow step, using only species present in or before it.

128. How does a homogeneous catalyst differ from a heterogeneous catalyst?
     • A homogeneous catalyst is always a metal
     • A homogeneous catalyst raises activation energy
     • A homogeneous catalyst is in the same phase as the reactants
     • A homogeneous catalyst is consumed in the reaction

     Correct answer: A homogeneous catalyst is in the same phase as the reactants

     In the same phase as the reactants is the correct answer. A homogeneous catalyst exists in the same physical phase as the reactants, while a heterogeneous catalyst is in a different phase, such as a solid in a gas reaction.

129. A catalyst appears in a reaction mechanism in what pattern?
     • It is produced once and never used
     • It is consumed in an early step and regenerated in a later step
     • It only appears in the overall equation
     • It permanently becomes a product

     Correct answer: It is consumed in an early step and regenerated in a later step

     Consumed early and regenerated later is the correct answer. A catalyst is used up in one step of the mechanism and reformed in a subsequent step, so it does not appear in the overall balanced equation.

130. The Maxwell-Boltzmann distribution of molecular energies changes in what way when temperature increases?
     • The curve shifts so more molecules have high energies
     • The curve narrows and shifts to lower energies
     • The total area under the curve increases
     • The activation energy moves to the left

     Correct answer: The curve shifts so more molecules have high energies

     More molecules have high energies is the correct answer. Raising temperature broadens and flattens the Maxwell-Boltzmann curve, shifting it so a larger fraction of molecules exceed the activation energy.

131. For a first-order reaction, after one half-life has passed, what fraction of the original reactant remains?
     • Three quarters
     • One half
     • One quarter
     • All of it

     Correct answer: One half

     One half is the correct answer. By definition, one half-life is the time required for the reactant concentration to drop to one-half of its initial value.

132. The molecularity of an elementary step that involves a single reactant molecule decomposing is described as what?
     • Bimolecular
     • Termolecular
     • Unimolecular
     • Zero molecular

     Correct answer: Unimolecular

     Unimolecular is the correct answer. An elementary step in which one molecule reacts on its own, such as a decomposition, is called unimolecular.

133. An elementary step in which two molecules collide and react is described by which term?
     • Unimolecular
     • Termolecular
     • Zero molecular
     • Bimolecular

     Correct answer: Bimolecular

     Bimolecular is the correct answer. An elementary step involving the collision of two reactant species is called bimolecular, the most common type of elementary step.

134. For an exothermic reaction, how does the activation energy of the forward reaction compare to that of the reverse reaction?
     • The forward activation energy is smaller than the reverse
     • The forward and reverse activation energies are equal
     • The forward activation energy is larger than the reverse
     • The reverse reaction has no activation energy

     Correct answer: The forward activation energy is smaller than the reverse

     The forward activation energy is smaller is the correct answer. In an exothermic reaction the products lie below the reactants, so the barrier from reactants to the peak is smaller than the barrier from products back to the peak.

135. Why are termolecular elementary steps very rare in reaction mechanisms?
     • Three molecules repel one another completely
     • Three molecules colliding simultaneously with proper orientation is highly improbable
     • Three molecules always form a catalyst
     • Three molecules cannot have activation energy

     Correct answer: Three molecules colliding simultaneously with proper orientation is highly improbable

     A simultaneous three-body collision is highly improbable is the correct answer. Termolecular steps require three particles to collide at the same instant with correct orientation, an event so unlikely that such steps are uncommon.

136. When the rate-determining step is not the first step and involves an intermediate, the rate law must be rewritten to do what?
     • Include the catalyst concentration only
     • Ignore all fast steps
     • Express the intermediate in terms of reactants using the prior fast equilibrium
     • Use the product concentrations exclusively

     Correct answer: Express the intermediate in terms of reactants using the prior fast equilibrium

     Express the intermediate in terms of reactants is the correct answer. Because intermediates cannot appear in the final rate law, a preceding fast equilibrium is used to substitute the intermediate with measurable reactant concentrations.

137. In thermochemistry, the part of the universe that is being studied, such as the reacting chemicals, is referred to by which term?
     • The system
     • The surroundings
     • The boundary
     • The reservoir

     Correct answer: The system

     The system is the correct answer. In thermochemistry the system is the specific portion of the universe under study, while everything else, such as the container and air, is collectively called the surroundings.

138. Which statement best distinguishes heat from temperature?
     • Heat and temperature are identical quantities
     • Heat is the average kinetic energy of particles while temperature is total energy
     • Heat is energy transferred due to a temperature difference, while temperature measures average kinetic energy of particles
     • Temperature is always larger than heat

     Correct answer: Heat is energy transferred due to a temperature difference, while temperature measures average kinetic energy of particles

     Heat is transferred energy and temperature is average kinetic energy is the correct answer. Heat is the flow of thermal energy between objects at different temperatures, whereas temperature is a measure of the average kinetic energy of the particles in a sample.

139. When a system absorbs heat from its surroundings, the sign convention assigns the value of q for the system as what?
     • Always zero
     • Equal to the mass
     • Positive
     • Negative

     Correct answer: Positive

     Positive is the correct answer. By convention, heat flowing into the system is given a positive q because the system gains thermal energy, which is the case for an endothermic process.

140. Two metal blocks at different temperatures are placed in contact in an insulated box. Heat will flow in which direction?
     • From the cooler block to the warmer block
     • In both directions equally so nothing changes
     • No heat flows because the box is insulated
     • From the warmer block to the cooler block

     Correct answer: From the warmer block to the cooler block

     From the warmer block to the cooler block is the correct answer. Thermal energy spontaneously transfers from the higher-temperature object to the lower-temperature object until both reach the same temperature.

141. Heat capacity differs from specific heat capacity in that heat capacity refers to the energy needed to raise the temperature of what?
     • One gram of a substance by one degree
     • One mole of a substance by one degree
     • An entire given object or sample by one degree
     • One liter of a gas by one degree

     Correct answer: An entire given object or sample by one degree

     An entire object or sample by one degree is the correct answer. Heat capacity is the energy required to raise the temperature of a whole given sample by one degree, while specific heat capacity is stated per gram of substance.

142. If equal masses of two metals receive the same amount of heat, the metal with the lower specific heat will experience what?
     • A larger temperature increase
     • A smaller temperature increase
     • No change in temperature
     • A decrease in temperature

     Correct answer: A larger temperature increase

     A larger temperature increase is the correct answer. A lower specific heat means less energy is needed per degree of warming, so for the same heat input that metal rises more in temperature than one with a higher specific heat.

143. In the heat equation q equals m times c times the temperature change, what units are most commonly used for the heat q in AP Chemistry?
     • Grams
     • Degrees Celsius
     • Moles
     • Joules

     Correct answer: Joules

     Joules is the correct answer. The quantity of heat q is an energy term, so it is expressed in joules (or kilojoules), with mass in grams, specific heat in joules per gram per degree, and temperature change in degrees.

144. The enthalpy of solution describes the heat change that occurs when which process takes place?
     • A solid melts into a liquid
     • A gas is compressed at constant temperature
     • A solute dissolves in a solvent to form a solution
     • An element forms from a compound

     Correct answer: A solute dissolves in a solvent to form a solution

     A solute dissolves in a solvent is the correct answer. The enthalpy of solution is the overall heat absorbed or released when a solute is fully dissolved in a solvent to make a solution.

145. When an instant cold pack is activated and feels cold, the dissolving of the salt inside is best described as what type of process?
     • Exothermic, releasing heat
     • A combustion process
     • A process with no enthalpy change
     • Endothermic, absorbing heat

     Correct answer: Endothermic, absorbing heat

     Endothermic is the correct answer. The cold pack absorbs heat from its surroundings as the salt dissolves, lowering the temperature, which is the defining feature of an endothermic dissolving process.

146. For the reaction shown with a molar enthalpy of negative 286 kilojoules per mole, what does this value indicate about the reaction?
     • It absorbs 286 kilojoules per mole of reaction
     • It requires 286 grams of reactant
     • It releases 286 kilojoules per mole of reaction
     • It has no measurable heat change

     Correct answer: It releases 286 kilojoules per mole of reaction

     It releases 286 kilojoules per mole is the correct answer. A negative molar enthalpy means the reaction gives off that quantity of energy to the surroundings per mole, identifying it as exothermic.

147. At the particulate level, the energy released by an exothermic reaction ultimately appears as what in the surroundings?
     • Increased kinetic motion of surrounding particles
     • New chemical bonds in the surroundings
     • Loss of mass from the system
     • A decrease in the number of particles

     Correct answer: Increased kinetic motion of surrounding particles

     Increased kinetic motion of surrounding particles is the correct answer. Energy released by the reaction transfers to surrounding particles, raising their average kinetic energy and therefore the temperature of the surroundings.

148. During boiling at the normal boiling point, the temperature of a pure liquid stays constant even as heat is added because the energy is used to do what?
     • Raise the average kinetic energy of the molecules
     • Form new covalent bonds within molecules
     • Separate molecules by overcoming intermolecular attractions
     • Increase the pressure of the system

     Correct answer: Separate molecules by overcoming intermolecular attractions

     Separate molecules by overcoming intermolecular attractions is the correct answer. During a phase change the added energy works against intermolecular forces to convert liquid to vapor rather than increasing kinetic energy, so temperature holds steady.

149. On a heating curve, the flat plateaus correspond to which events?
     • Steady warming of a single phase
     • Rapid cooling of the sample
     • The destruction of chemical bonds within molecules
     • Phase changes occurring at constant temperature

     Correct answer: Phase changes occurring at constant temperature

     Phase changes at constant temperature is the correct answer. The horizontal segments of a heating curve represent melting or boiling, where added heat drives the phase change while the temperature remains unchanged.

150. A 50.0 gram sample of metal at 90 degrees Celsius is dropped into cool water. As the system reaches equilibrium, the magnitude of heat lost by the metal compares to the heat gained by the water in what way, assuming no losses?
     • The metal loses much less heat than the water gains
     • The metal gains heat from the water
     • The metal loses an equal magnitude of heat to what the water gains
     • No heat is exchanged between them

     Correct answer: The metal loses an equal magnitude of heat to what the water gains

     Equal magnitude of heat is the correct answer. With no heat lost to the environment, energy conservation requires that the heat released by the cooling metal equals in magnitude the heat absorbed by the warming water.

151. In calorimetry, why is a styrofoam coffee-cup calorimeter used for many solution reactions?
     • It conducts heat rapidly to the environment
     • It increases the pressure on the reaction
     • It changes the specific heat of the water
     • It insulates the contents to minimize heat exchange with the surroundings

     Correct answer: It insulates the contents to minimize heat exchange with the surroundings

     It insulates to minimize heat exchange is the correct answer. The styrofoam acts as an insulator, limiting heat flow between the reaction mixture and the outside so the measured temperature change reflects the reaction itself.

152. To convert a heat measured in joules from a calorimetry experiment into a molar enthalpy in kilojoules per mole, a student should do what?
     • Multiply joules by the number of moles
     • Add the mass of water to the heat
     • Divide the heat by the moles of the limiting reactant and convert to kilojoules
     • Subtract the temperature change from the heat

     Correct answer: Divide the heat by the moles of the limiting reactant and convert to kilojoules

     Divide by moles and convert to kilojoules is the correct answer. Dividing the total heat by the moles of reactant gives energy per mole, and converting joules to kilojoules yields the molar enthalpy in kilojoules per mole.

153. Given the standard enthalpies of formation of the reactants and products, the enthalpy of a reaction is calculated as the sum of the formation enthalpies of the products minus what?
     • The sum of the formation enthalpies of the reactants
     • The temperature of the reaction
     • The mass of the products
     • The number of moles of catalyst

     Correct answer: The sum of the formation enthalpies of the reactants

     The sum of the formation enthalpies of the reactants is the correct answer. The reaction enthalpy equals the total standard formation enthalpies of the products minus those of the reactants, each weighted by its coefficient.

154. When applying the formula that uses enthalpies of formation, each compound's formation value must be multiplied by what before summing?
     • Its molar mass
     • Its coefficient from the balanced equation
     • Its specific heat
     • The reaction temperature

     Correct answer: Its coefficient from the balanced equation

     Its coefficient from the balanced equation is the correct answer. Because enthalpy is extensive, each substance's standard formation enthalpy is scaled by its stoichiometric coefficient before the products and reactants are summed.

155. Why do bond enthalpy calculations give only an approximate value for a reaction's enthalpy?
     • Bond enthalpies are exact for every molecule
     • Bonds do not store any energy
     • Bond enthalpies ignore the products entirely
     • Bond enthalpies are average values that vary slightly between different molecules

     Correct answer: Bond enthalpies are average values that vary slightly between different molecules

     They are average values that vary between molecules is the correct answer. Tabulated bond enthalpies are averages taken over many compounds, so a calculation using them yields an estimate rather than the exact reaction enthalpy.

156. If breaking all the bonds in the reactants requires more energy than is released when product bonds form, the reaction is classified as what?
     • Exothermic
     • Endothermic
     • Thermally neutral
     • Catalyzed

     Correct answer: Endothermic

     Endothermic is the correct answer. When more energy goes into breaking reactant bonds than is recovered from forming product bonds, the net enthalpy change is positive and the reaction absorbs energy overall.

157. A scientist needs the enthalpy of a reaction that is difficult to measure directly. Using known enthalpies for related reactions to calculate it relies on which principle?
     • The ideal gas law
     • Avogadro's hypothesis
     • Hess's law
     • The law of definite proportions

     Correct answer: Hess's law

     Hess's law is the correct answer. Because enthalpy is a state function, Hess's law lets a hard-to-measure reaction enthalpy be obtained by adding the enthalpies of a chosen set of reactions that combine to give the target reaction.

158. A reaction releases 4.18 kilojoules of heat into 100.0 grams of water. Given water's specific heat of 4.18 joules per gram per degree, the water temperature rises by approximately how much?
     • 1 degree Celsius
     • 10 degrees Celsius
     • 100 degrees Celsius
     • 0.1 degrees Celsius

     Correct answer: 10 degrees Celsius

     10 degrees Celsius is the correct answer. Rearranging q equals m times c times the temperature change gives 4180 joules divided by the product of 100 grams and 4.18 joules per gram per degree, which equals about 10 degrees.

159. For the reaction in which dinitrogen tetroxide decomposes into nitrogen dioxide, the equilibrium constant Kc has units that depend on what?
     • The difference in moles of gaseous products and reactants
     • The temperature only
     • The total pressure
     • Nothing, Kc is always unitless in AP chemistry

     Correct answer: Nothing, Kc is always unitless in AP chemistry

     Unitless is the correct answer. In AP chemistry equilibrium constants are treated as dimensionless because each concentration or pressure term is divided by a standard reference state, so no units are reported.

160. If the coefficients of a balanced equilibrium reaction are all multiplied by two, how does the new equilibrium constant relate to the original K?
     • It becomes K squared
     • It is unchanged
     • It is doubled
     • It becomes one half of K

     Correct answer: It becomes K squared

     K squared is the correct answer. Multiplying every coefficient by a factor n raises the equilibrium constant to the nth power, so doubling all coefficients squares the original K.

161. Two reactions are added together to give an overall reaction. The equilibrium constant for the overall reaction equals what?
     • The sum of the two individual constants
     • The product of the two individual constants
     • The difference of the two constants
     • The average of the two constants

     Correct answer: The product of the two individual constants

     The product of the two constants is the correct answer. When reactions are added to obtain an overall reaction, their equilibrium constants are multiplied together to give the overall constant.

162. At a given temperature, a reaction mixture has Q greater than K. Which statement describes what happens next?
     • The reaction shifts forward to make more product
     • No net reaction occurs
     • The reaction shifts in reverse to form more reactant
     • The temperature must increase

     Correct answer: The reaction shifts in reverse to form more reactant

     Shifts in reverse is the correct answer. When Q exceeds K there is too much product relative to equilibrium, so the net reaction proceeds backward, converting product to reactant until Q equals K.

163. For a sparingly soluble salt that dissolves to give two ions in a one-to-one ratio, the molar solubility s relates to Ksp by which expression?
     • Ksp equals two times s
     • Ksp equals s cubed
     • Ksp equals s divided by two
     • Ksp equals s squared

     Correct answer: Ksp equals s squared

     Ksp equals s squared is the correct answer. For a salt producing one cation and one anion each at concentration s, the solubility product is the product of the two ion concentrations, which is s times s, or s squared.

164. In an ICE table problem, the small x approximation is most valid when which condition holds?
     • The equilibrium constant is very small relative to the initial concentration
     • The equilibrium constant is very large
     • The temperature is high
     • The reaction is exothermic

     Correct answer: The equilibrium constant is very small relative to the initial concentration

     Very small K relative to initial concentration is the correct answer. When K is small, the change x is tiny compared with the initial concentration, so subtracting x can be neglected to simplify the algebra.

165. A reaction vessel is compressed to half its volume at constant temperature. For an equilibrium with equal moles of gas on both sides, the equilibrium position will do what?
     • Shift toward products
     • Not shift, because both sides have equal moles of gas
     • Shift toward reactants
     • Convert all reactant to product

     Correct answer: Not shift, because both sides have equal moles of gas

     Not shift is the correct answer. A volume change only shifts equilibrium when the two sides differ in moles of gas, so with equal moles on each side the position is unaffected.

166. The relationship between Kp and Kc for a gas-phase reaction involves which quantity raised to the change in moles of gas?
     • The temperature in Celsius
     • The product RT, the gas constant times absolute temperature
     • The total pressure
     • Avogadro's number

     Correct answer: The product RT, the gas constant times absolute temperature

     RT is the correct answer. Kp equals Kc times the quantity RT raised to the change in moles of gas, where R is the gas constant and T is the absolute temperature.

167. For a reaction with a change in moles of gas equal to zero, how do Kp and Kc compare?
     • Kp is larger than Kc
     • They are equal
     • Kp is smaller than Kc
     • Kp is the reciprocal of Kc

     Correct answer: They are equal

     They are equal is the correct answer. When the moles of gas do not change, the factor RT is raised to the zero power, which equals one, making Kp and Kc numerically identical.

168. A precipitate is predicted to form in a mixed solution when the ion product Q is in what relationship with Ksp?
     • Q greater than Ksp
     • Q equal to Ksp
     • Q less than Ksp
     • Q equal to zero

     Correct answer: Q greater than Ksp

     Q greater than Ksp is the correct answer. When the calculated ion product exceeds the solubility product, the solution is supersaturated and a precipitate forms until Q falls back to Ksp.

169. If Q equals Ksp for a solution of a slightly soluble salt, the solution is described as which of the following?
     • Unsaturated
     • Exactly saturated and at equilibrium
     • Supersaturated
     • Pure solvent

     Correct answer: Exactly saturated and at equilibrium

     Exactly saturated and at equilibrium is the correct answer. When the ion product equals the solubility product, the dissolved ions are in equilibrium with the solid, defining a saturated solution.

170. For an endothermic reaction, increasing the temperature has what effect on the magnitude of the equilibrium constant K?
     • K decreases
     • K stays exactly the same
     • K increases
     • K becomes negative

     Correct answer: K increases

     K increases is the correct answer. For an endothermic reaction, raising the temperature favors product formation, which increases the value of the equilibrium constant.

171. Which factor is the only one that actually changes the numerical value of the equilibrium constant for a given reaction?
     • Concentration
     • Pressure
     • Temperature
     • Adding a catalyst

     Correct answer: Temperature

     Temperature is the correct answer. Changes in concentration, pressure, or catalyst shift the position of equilibrium but only a change in temperature alters the value of the equilibrium constant itself.

172. Consider the equilibrium between a solid carbonate and its decomposition into a solid oxide plus carbon dioxide gas. The equilibrium constant expression contains only what?
     • The two solids
     • The partial pressure of carbon dioxide
     • All three species
     • The temperature

     Correct answer: The partial pressure of carbon dioxide

     The partial pressure of carbon dioxide is the correct answer. Pure solids are excluded from the expression, so only the gaseous carbon dioxide term remains in the equilibrium constant.

173. For a reaction quotient calculated from initial concentrations, finding that Q equals K means which of the following?
     • The mixture is already at equilibrium and will not shift
     • The reaction will shift forward
     • The reaction will shift backward
     • The reaction is impossible

     Correct answer: The mixture is already at equilibrium and will not shift

     Already at equilibrium is the correct answer. When Q equals K, the concentrations already satisfy the equilibrium condition, so there is no net shift in either direction.

174. A buffer-like equilibrium resists change when a small amount of one species is added. This behavior is a direct consequence of which principle?
     • The ideal gas law
     • Le Chatelier's principle
     • Hess's law
     • The aufbau principle

     Correct answer: Le Chatelier's principle

     Le Chatelier's principle is the correct answer. The principle states that a system at equilibrium responds to a stress by shifting to partially counteract that stress, which underlies the resistance to change.

175. For the synthesis of ammonia from nitrogen and hydrogen gases, which change would shift the equilibrium toward more ammonia?
     • Decreasing the total pressure
     • Removing nitrogen gas
     • Increasing the total pressure by decreasing volume
     • Adding an inert gas at constant volume

     Correct answer: Increasing the total pressure by decreasing volume

     Increasing pressure by decreasing volume is the correct answer. The product ammonia side has fewer moles of gas, so raising pressure shifts the equilibrium toward the side with fewer gas moles, producing more ammonia.

176. When solid silver chloride is placed in pure water, the dissolution stops appearing to change once which condition is reached?
     • All the solid has dissolved
     • The solution becomes saturated and dissolution equals precipitation
     • The water evaporates
     • The temperature drops to zero

     Correct answer: The solution becomes saturated and dissolution equals precipitation

     Saturated with dissolution equal to precipitation is the correct answer. At saturation the rate of ions leaving the solid equals the rate of ions rejoining it, so no net change is observed though both processes continue.

177. For a generic equilibrium, doubling the concentration of a reactant while holding temperature constant changes Q in what way before the system re-equilibrates?
     • Q decreases
     • Q stays equal to K
     • Q increases
     • Q becomes zero

     Correct answer: Q decreases

     Q decreases is the correct answer. Increasing a reactant concentration enlarges the denominator of the reaction quotient, which lowers Q below K and drives the reaction forward.

178. The equilibrium constant Kc for a reaction is reported as a value far greater than one. This implies the position of equilibrium lies where?
     • Far to the left toward reactants
     • Far to the right toward products
     • Exactly in the middle
     • Cannot be determined from K

     Correct answer: Far to the right toward products

     Far to the right toward products is the correct answer. A Kc much larger than one means product concentrations greatly exceed reactant concentrations at equilibrium, placing the position toward the products.

179. Why does adding a noble gas to a constant-volume equilibrium mixture leave the value of Q unchanged?
     • It reacts with the products
     • It changes the temperature
     • It does not alter the partial pressures or concentrations of the reacting gases
     • It is consumed by the reactants

     Correct answer: It does not alter the partial pressures or concentrations of the reacting gases

     It does not alter partial pressures or concentrations is the correct answer. At constant volume the noble gas raises total pressure but the individual concentrations and partial pressures of the reacting species stay the same, so Q is unchanged.

180. For the equilibrium expression of an aqueous reaction, which concentration is treated as effectively constant and therefore excluded?
     • The concentration of a dissolved gas
     • The molarity of a dilute solute
     • The concentration of the pure liquid water solvent
     • The concentration of an aqueous ion

     Correct answer: The concentration of the pure liquid water solvent

     The pure liquid water solvent is the correct answer. In dilute aqueous solutions the concentration of the water solvent is essentially constant, so it is omitted from the equilibrium constant expression.

181. In the reaction where ammonia gains a proton from water, which species acts as the Bronsted-Lowry base?
     • Ammonia
     • Water
     • The hydroxide ion produced
     • The ammonium ion produced

     Correct answer: Ammonia

     Ammonia is the correct answer. Ammonia accepts a proton from water to form the ammonium ion, so by the Bronsted-Lowry definition ammonia is the proton acceptor and therefore the base.

182. Which pairing correctly identifies a conjugate acid-base pair that differs by exactly one proton?
     • Hydrochloric acid and hydroxide ion
     • Hydronium ion and hydroxide ion
     • Sulfuric acid and chloride ion
     • Ammonium ion and ammonia

     Correct answer: Ammonium ion and ammonia

     Ammonium ion and ammonia is the correct answer. The ammonium ion and ammonia differ by a single hydrogen ion, so they form a true conjugate acid-base pair, with the ammonium ion as the acid.

183. The stronger an acid is, the weaker its conjugate base tends to be. This trend exists because a strong acid does what?
     • Holds onto its proton tightly
     • Readily gives up its proton, leaving a base with little tendency to reaccept it
     • Forms a conjugate base that strongly attracts protons
     • Cannot form a conjugate base

     Correct answer: Readily gives up its proton, leaving a base with little tendency to reaccept it

     Readily gives up its proton is the correct answer. Because a strong acid donates its proton almost completely, the resulting conjugate base has very little attraction for protons and is therefore weak.

184. What is the approximate hydrogen ion concentration of a solution with a pH of 5?
     • $1.0 \times 10^{-9}$ molar
     • $1.0 \times 10^{5}$ molar
     • $1.0 \times 10^{-5}$ molar
     • 5.0 molar

     Correct answer: $1.0 \times 10^{-5}$ molar

     $1.0 \times 10^{-5}$ molar is the correct answer. Because pH is the negative logarithm of the hydrogen ion concentration, a pH of 5 corresponds to a hydrogen ion concentration of $10^{-5}$.

185. A solution has a hydroxide ion concentration of $1.0 \times 10^{-3}$ molar at 25 degrees Celsius. What is its pH?
     • 3
     • 11
     • 7
     • 1

     Correct answer: 11

     11 is the correct answer. The pOH equals the negative logarithm of the hydroxide concentration, which is 3, and subtracting 3 from 14 gives a pH of 11.

186. For a weak acid with a Ka of $1.0 \times 10^{-5}$, what is its pKa?
     • 5
     • 9
     • Minus 5
     • 14

     Correct answer: 5

     5 is the correct answer. pKa is the negative logarithm of Ka, so the negative logarithm of $10^{-5}$ equals 5.

187. Which of the following weak acids is the strongest, based on its Ka value?
     • An acid with Ka of $1.8 \times 10^{-5}$
     • An acid with Ka of $6.2 \times 10^{-8}$
     • An acid with Ka of $7.5 \times 10^{-3}$
     • An acid with Ka of $4.0 \times 10^{-10}$

     Correct answer: An acid with Ka of $7.5 \times 10^{-3}$

     The acid with Ka of $7.5 \times 10^{-3}$ is the correct answer. A larger Ka indicates more complete ionization, and among the choices this value is the largest, so that acid is the strongest.

188. To prepare a buffer with a pH equal to the pKa of the weak acid, what ratio of conjugate base to weak acid is required?
     • A ratio of one to one
     • A ratio of ten to one
     • A ratio of one to ten
     • A ratio of one hundred to one

     Correct answer: A ratio of one to one

     A ratio of one to one is the correct answer. When the conjugate base and weak acid are present in equal amounts, the logarithm term in the Henderson-Hasselbalch equation is zero, so the pH equals the pKa.

189. For a buffer where the conjugate base concentration is ten times the weak acid concentration, the pH relative to the pKa is what?
     • One unit below the pKa
     • Equal to the pKa
     • One unit above the pKa
     • Two units above the pKa

     Correct answer: One unit above the pKa

     One unit above the pKa is the correct answer. The logarithm of a ten to one ratio of base to acid equals one, so the Henderson-Hasselbalch equation gives a pH that is one unit higher than the pKa.

190. During the titration of a weak acid with a strong base, the region before the equivalence point where pH changes slowly is called the what?
     • Buffer region
     • Equivalence point
     • Endpoint
     • Neutral region

     Correct answer: Buffer region

     Buffer region is the correct answer. Before the equivalence point the solution contains both the weak acid and its conjugate base, forming a buffer that resists pH change and produces a gently sloping section of the curve.

191. At the start of a titration of a strong acid with a strong base, the pH of the solution is what?
     • High
     • Low
     • Exactly 7
     • Exactly 14

     Correct answer: Low

     Low is the correct answer. Before any base is added the solution contains only the strong acid, so the hydrogen ion concentration is high and the pH is low.

192. How many moles of hydroxide ion are required to exactly neutralize 0.50 mol of a monoprotic acid?
     • 0.25 mol
     • 0.50 mol
     • 1.0 mol
     • 2.0 mol

     Correct answer: 0.50 mol

     0.50 mol is the correct answer. A monoprotic acid donates one proton per molecule, so neutralizing 0.50 mol of the acid requires an equal 0.50 mol of hydroxide ion in a one to one reaction.

193. A 25.0 milliliter sample of acid requires 25.0 milliliters of 0.100 molar base to reach the equivalence point. If the reaction is one to one, what is the acid concentration?
     • 0.0500 molar
     • 0.100 molar
     • 0.200 molar
     • 0.250 molar

     Correct answer: 0.100 molar

     0.100 molar is the correct answer. Equal volumes and a one to one mole ratio mean the acid and base concentrations are equal, so the acid concentration matches the base at 0.100 molar.

194. Why does phenolphthalein, which changes color around pH 8 to 10, work well for a weak acid titrated with a strong base?
     • Its transition range matches the equivalence point pH, which is above 7
     • It changes color at low pH
     • It works only for strong acids
     • Its color never changes

     Correct answer: Its transition range matches the equivalence point pH, which is above 7

     Its transition range matches the equivalence point pH is the correct answer. A weak acid titrated with a strong base has an equivalence point above 7, so an indicator like phenolphthalein that changes near pH 8 to 10 signals the endpoint accurately.

195. The autoionization of water produces which two ions?
     • Hydrogen gas and oxygen gas
     • Hydronium ions and hydroxide ions
     • Sodium ions and chloride ions
     • Oxide ions and hydrogen ions

     Correct answer: Hydronium ions and hydroxide ions

     Hydronium ions and hydroxide ions is the correct answer. In autoionization one water molecule transfers a proton to another, generating a hydronium ion and a hydroxide ion in equal amounts.

196. In a 0.10 molar solution of a strong monoprotic acid, what is the approximate concentration of hydroxide ions at 25 degrees Celsius?
     • 0.10 molar
     • $1.0 \times 10^{-7}$ molar
     • $1.0 \times 10^{-13}$ molar
     • $1.0 \times 10^{-1}$ molar

     Correct answer: $1.0 \times 10^{-13}$ molar

     $1.0 \times 10^{-13}$ molar is the correct answer. The hydrogen ion concentration is 0.10 molar, and dividing the ion product of water by this value gives a hydroxide concentration of about $10^{-13}$.

197. Which structural factor generally makes an oxoacid stronger as more oxygen atoms are bonded to the central atom?
     • The added oxygens weaken the molecule
     • The added oxygens pull electron density away, weakening the oxygen-hydrogen bond
     • The added oxygens add protons
     • The added oxygens make the acid a base

     Correct answer: The added oxygens pull electron density away, weakening the oxygen-hydrogen bond

     The added oxygens pull electron density away is the correct answer. Additional electronegative oxygen atoms draw electron density from the oxygen-hydrogen bond, weakening it so the proton is released more easily and the acid is stronger.

198. In the series of binary acids of the halogens, hydroiodic acid is stronger than hydrofluoric acid mainly because of what?
     • The stronger oxygen-hydrogen bond in hydroiodic acid
     • The weaker hydrogen-iodine bond, which breaks more easily
     • The greater electronegativity of iodine
     • The smaller size of the iodine atom

     Correct answer: The weaker hydrogen-iodine bond, which breaks more easily

     The weaker hydrogen-iodine bond is the correct answer. The larger iodine atom forms a longer, weaker bond to hydrogen than fluorine does, so hydroiodic acid releases its proton more readily and is the stronger acid.

199. A buffer is made and then diluted with a large volume of pure water. How does the pH change?
     • The pH stays nearly the same because the ratio of base to acid is unchanged
     • The pH rises sharply
     • The pH drops to that of pure water
     • The pH becomes exactly 7

     Correct answer: The pH stays nearly the same because the ratio of base to acid is unchanged

     The pH stays nearly the same is the correct answer. Diluting a buffer changes both the conjugate base and weak acid concentrations by the same factor, so their ratio and therefore the pH remain nearly constant, although buffer capacity drops.

200. Which combination would NOT form an effective buffer solution?
     • Acetic acid and sodium acetate
     • Ammonia and ammonium chloride
     • Hydrochloric acid and sodium chloride
     • A weak acid and a salt of its conjugate base

     Correct answer: Hydrochloric acid and sodium chloride

     Hydrochloric acid and sodium chloride is the correct answer. A buffer requires a weak acid and its conjugate base, but hydrochloric acid is a strong acid and chloride is its essentially nonbasic conjugate, so this pair cannot buffer.

201. What does the percent ionization of a weak acid tell you?
     • The fraction of acid molecules that dissociate into ions
     • The mass of the acid
     • The total volume of solution
     • The boiling point elevation

     Correct answer: The fraction of acid molecules that dissociate into ions

     The fraction of acid molecules that dissociate is the correct answer. Percent ionization expresses the ratio of ionized acid to the original acid concentration as a percentage, indicating how completely the weak acid dissociates.

202. For a weak acid, the approximate hydrogen ion concentration can be estimated using which relationship when ionization is small?
     • The hydrogen ion concentration equals $K_a$
     • The hydrogen ion concentration equals the initial concentration
     • The hydrogen ion concentration equals $K_w$
     • The hydrogen ion concentration equals $\sqrt{K_a \times \text{initial acid concentration}}$

     Correct answer: The hydrogen ion concentration equals $\sqrt{K_a \times \text{initial acid concentration}}$

     $\sqrt{K_a \times \text{initial concentration}}$ is the correct answer. For a weak acid with small ionization, the equilibrium expression simplifies so the hydrogen ion concentration approximately equals $\sqrt{K_a \times \text{initial acid concentration}}$.

203. When a strong base is added beyond the equivalence point in a strong acid titration, what controls the pH?
     • The remaining acid
     • The excess hydroxide ions from the added base
     • The salt formed
     • The indicator

     Correct answer: The excess hydroxide ions from the added base

     The excess hydroxide ions is the correct answer. After the equivalence point all the acid is neutralized, so the leftover hydroxide from the added strong base determines the high pH.

204. A diprotic acid is titrated with a strong base. How many equivalence points appear on the titration curve?
     • One
     • Two
     • Three
     • None

     Correct answer: Two

     Two is the correct answer. A diprotic acid has two ionizable protons that are neutralized in two successive steps, so its titration curve shows two distinct equivalence points.

205. Which statement best explains why the pH of a buffer is relatively insensitive to small additions of base?
     • The weak acid component reacts with the added base to limit the pH increase
     • The buffer has no acid
     • The water boils off the base
     • The buffer increases its own concentration

     Correct answer: The weak acid component reacts with the added base to limit the pH increase

     The weak acid component reacts with the added base is the correct answer. When base is added, the weak acid in the buffer neutralizes most of it, converting to conjugate base and keeping the pH change small.

206. At the equivalence point of a titration, which condition is always true?
     • The pH is exactly 7
     • The moles of added titrant equal the moles needed to react with all the analyte
     • The indicator has not yet changed color
     • No reaction has occurred

     Correct answer: The moles of added titrant equal the moles needed to react with all the analyte

     The moles of titrant equal the moles needed to react with all the analyte is the correct answer. The equivalence point is defined by stoichiometric completion of the reaction, regardless of whether the resulting pH is acidic, neutral, or basic.

207. Sodium carbonate dissolved in water produces a solution that is basic. Which process is responsible?
     • The sodium ion donates protons
     • The carbonate ion accepts protons from water, releasing hydroxide ions
     • The water decomposes into oxygen
     • The salt does not dissolve

     Correct answer: The carbonate ion accepts protons from water, releasing hydroxide ions

     The carbonate ion accepts protons from water is the correct answer. As the conjugate base of a weak acid, the carbonate ion hydrolyzes by taking protons from water, generating hydroxide ions and making the solution basic.

208. Which salt would dissolve to give a nearly neutral solution?
     • Ammonium chloride
     • Sodium acetate
     • Sodium carbonate
     • Sodium chloride

     Correct answer: Sodium chloride

     Sodium chloride is the correct answer. Both the sodium ion and the chloride ion come from a strong base and a strong acid respectively, so neither hydrolyzes appreciably and the solution remains nearly neutral.

209. As a weak acid solution is progressively diluted, its percent ionization does what?
     • Increases
     • Decreases
     • Stays exactly constant
     • Drops to zero

     Correct answer: Increases

     Increases is the correct answer. Diluting a weak acid shifts the ionization equilibrium toward more dissociation, so even though the hydrogen ion concentration falls, the percent of acid molecules that ionize increases.

210. The hydronium ion concentration in a neutral solution at 25 degrees Celsius is which value?
     • $1.0 \times 10^{-14}$ molar
     • $1.0 \times 10^{-7}$ molar
     • 1.0 molar
     • Zero

     Correct answer: $1.0 \times 10^{-7}$ molar

     $1.0 \times 10^{-7}$ molar is the correct answer. In a neutral solution the hydronium and hydroxide concentrations are equal, and since their product is $10^{-14}$, each must be $10^{-7}$.

211. If a solution has a pOH of 9 at 25 degrees Celsius, the solution is best described as what?
     • Acidic
     • Basic
     • Neutral
     • Nonaqueous

     Correct answer: Acidic

     Acidic is the correct answer. A pOH of 9 corresponds to a pH of 5, and since the pH is below 7 the solution is acidic.

212. Which factor increases the buffer capacity of a buffer solution?
     • A very dilute solution
     • Using a strong acid instead of a weak acid
     • Removing the conjugate base
     • Higher concentrations of the weak acid and conjugate base

     Correct answer: Higher concentrations of the weak acid and conjugate base

     Higher concentrations of the weak acid and conjugate base is the correct answer. A buffer with greater concentrations of both components can neutralize larger amounts of added acid or base before the pH changes significantly, giving it a higher capacity.

213. In a Lewis sense, a base is best described as a species that does what?
     • Donates a pair of electrons
     • Accepts a pair of electrons
     • Donates a proton
     • Accepts a proton only

     Correct answer: Donates a pair of electrons

     Donates a pair of electrons is the correct answer. The Lewis model defines a base as an electron-pair donor, a broader definition than the Bronsted-Lowry proton acceptor.

214. In a Lewis acid-base reaction, the Lewis acid does what?
     • Donates a proton
     • Accepts a pair of electrons to form a bond
     • Releases hydroxide ions
     • Donates a pair of electrons

     Correct answer: Accepts a pair of electrons to form a bond

     Accepts a pair of electrons is the correct answer. A Lewis acid is an electron-pair acceptor that forms a coordinate covalent bond with the electron pair supplied by the Lewis base.

215. What is the pH of a 1.0 molar solution of a strong monoprotic acid, assuming complete dissociation?
     • 1
     • 0
     • 7
     • Minus 1

     Correct answer: 0

     0 is the correct answer. Complete dissociation gives a hydrogen ion concentration of 1.0 molar, and the negative logarithm of 1 is 0.

216. Why is the pH at the equivalence point of a weak base titrated with a strong acid below 7?
     • The conjugate acid of the weak base remains and lowers the pH
     • The strong acid is in excess
     • No salt forms
     • The water becomes basic

     Correct answer: The conjugate acid of the weak base remains and lowers the pH

     The conjugate acid of the weak base remains is the correct answer. At the equivalence point the weak base has been converted to its conjugate acid, which donates protons to water and lowers the pH below 7.

217. For the conjugate base of a weak acid with Ka of $1.0 \times 10^{-5}$, what is the value of Kb at 25 degrees Celsius?
     • $1.0 \times 10^{-9}$
     • $1.0 \times 10^{-5}$
     • $1.0 \times 10^{-14}$
     • $1.0 \times 10^{9}$

     Correct answer: $1.0 \times 10^{-9}$

     $1.0 \times 10^{-9}$ is the correct answer. Since Ka times Kb equals the ion product of water, dividing $10^{-14}$ by the Ka of $10^{-5}$ yields a Kb of $10^{-9}$.

218. Which observation provides the best evidence that an unknown acid is weak rather than strong at a given concentration?
     • It has a measurable but incomplete degree of ionization, giving a higher pH than expected for full dissociation
     • It conducts electricity better than a strong acid at the same concentration
     • It cannot be neutralized
     • It has a pH of zero

     Correct answer: It has a measurable but incomplete degree of ionization, giving a higher pH than expected for full dissociation

     Incomplete ionization giving a higher pH is the correct answer. A weak acid only partially dissociates, so at the same concentration it produces fewer hydrogen ions and a higher pH than a fully dissociating strong acid would.

219. When equal volumes of a strong acid and a strong base of the same concentration are mixed, the resulting solution is what?
     • Strongly acidic
     • Strongly basic
     • Neutral
     • A buffer

     Correct answer: Neutral

     Neutral is the correct answer. Equal moles of a strong acid and a strong base react completely to form water and a salt that does not affect pH, so the mixture is neutral.

220. For a reaction in which the number of moles of gas increases from reactants to products, the standard entropy change is most likely what sign?
     • Positive
     • Negative
     • Zero
     • Undefined

     Correct answer: Positive

     Positive is the correct answer. Producing more moles of gas increases the dispersal of matter and the number of accessible microstates, so the standard entropy change is positive.

221. Standard entropy values are reported relative to a reference point established by the third law of thermodynamics, which assigns zero entropy to what?
     • Any gas at room temperature
     • A perfect crystalline solid at absolute zero
     • One mole of any liquid
     • An ideal solution at standard pressure

     Correct answer: A perfect crystalline solid at absolute zero

     A perfect crystalline solid at absolute zero is the correct answer. The third law of thermodynamics sets the entropy of a perfect crystal at absolute zero to zero, providing the baseline for absolute entropy values.

222. When dealing with thermodynamic favorability, a process driven primarily by a large decrease in enthalpy is best described as what kind of favorable?
     • Entropically favorable only
     • Unfavorable in every respect
     • Enthalpically favorable
     • Favorable only at high temperature

     Correct answer: Enthalpically favorable

     Enthalpically favorable is the correct answer. A large negative enthalpy change releases energy and makes the process enthalpically favorable, contributing a negative term to the free energy change.

223. For an endothermic reaction that becomes thermodynamically favorable only above a certain temperature, what must be true of the entropy change?
     • It must be negative
     • It must equal zero
     • It must equal the enthalpy change
     • It must be positive

     Correct answer: It must be positive

     It must be positive is the correct answer. When enthalpy is unfavorable, a positive entropy change allows the temperature-entropy term to eventually overcome it, making the reaction favorable above a threshold temperature.

224. The temperature at which a reaction switches between favorable and unfavorable can be estimated by setting the free energy change equal to what value?
     • Zero
     • The enthalpy change
     • The entropy change
     • The cell potential

     Correct answer: Zero

     Zero is the correct answer. At the crossover temperature the free energy change equals zero, so setting the Gibbs expression to zero and solving gives the temperature where favorability reverses.

225. In a standard hydrogen electrode used as a reference, the standard reduction potential is assigned what value?
     • Plus one volt
     • Exactly zero volts
     • Minus one volt
     • The same as the cathode

     Correct answer: Exactly zero volts

     Exactly zero volts is the correct answer. The standard hydrogen electrode is defined as the reference half-cell with a standard reduction potential of zero volts, against which all other potentials are measured.

226. To calculate the standard cell potential of a galvanic cell from half-reactions, you subtract the anode reduction potential from which quantity?
     • The standard entropy
     • The standard enthalpy
     • The cathode reduction potential
     • The equilibrium constant

     Correct answer: The cathode reduction potential

     The cathode reduction potential is the correct answer. The standard cell potential equals the cathode reduction potential minus the anode reduction potential, using both values as reduction potentials.

227. Two metals are placed in a galvanic cell. The metal with the more negative standard reduction potential will tend to do what?
     • Be reduced at the cathode
     • Remain unchanged
     • Act as the cathode
     • Be oxidized at the anode

     Correct answer: Be oxidized at the anode

     Be oxidized at the anode is the correct answer. The species with the more negative reduction potential is more easily oxidized, so it serves as the anode where oxidation occurs.

228. The relationship between standard cell potential and standard free energy change shows that as cell potential increases, the free energy change does what?
     • Becomes more negative
     • Becomes more positive
     • Stays constant
     • Approaches the entropy change

     Correct answer: Becomes more negative

     Becomes more negative is the correct answer. The free energy change is the negative product of the number of electrons, Faraday's constant, and the cell potential, so a larger positive potential gives a more negative free energy change.

229. In an electrolytic cell, the cathode is connected to which terminal of the external power source?
     • The positive terminal
     • The negative terminal
     • Neither terminal
     • The salt bridge

     Correct answer: The negative terminal

     The negative terminal is the correct answer. In an electrolytic cell the external power source pushes electrons to the cathode, so the cathode is connected to the negative terminal where reduction is forced to occur.

230. The quantity of charge carried by one mole of electrons is described by which constant?
     • Avogadro's number alone
     • The gas constant
     • Faraday's constant
     • The rate constant

     Correct answer: Faraday's constant

     Faraday's constant is the correct answer. Faraday's constant gives the magnitude of electric charge per mole of electrons and is used to convert between charge and moles of electrons in electrochemistry.

231. If a current is passed through a cell for a measured time, the total charge transferred is found by multiplying the current by what?
     • The voltage
     • The resistance
     • The molar mass
     • The elapsed time

     Correct answer: The elapsed time

     The elapsed time is the correct answer. Total charge equals current multiplied by the time the current flows, which is the first step in stoichiometric electrolysis calculations.

232. During the electroplating of a metal onto an object, the object to be plated serves as which electrode?
     • The cathode
     • The anode
     • The salt bridge
     • The reference electrode

     Correct answer: The cathode

     The cathode is the correct answer. In electroplating, metal ions are reduced and deposited onto the object, which therefore acts as the cathode where reduction takes place.

233. For a galvanic cell at nonstandard conditions, increasing the concentration of the reactant ions relative to products generally has what effect on the cell potential?
     • Decreases it below zero
     • Increases it above the standard value
     • Has no effect at all
     • Makes it equal to zero

     Correct answer: Increases it above the standard value

     Increases it above the standard value is the correct answer. By the Nernst relationship, raising reactant concentrations relative to products shifts the cell potential to a value higher than the standard cell potential.

234. When a galvanic cell has reached equilibrium, what is the value of its cell potential?
     • Its maximum positive value
     • Twice the standard potential
     • Zero
     • A large negative value

     Correct answer: Zero

     Zero is the correct answer. At equilibrium there is no net driving force for electron flow, so the measured cell potential of the dead cell is zero.

235. A reaction has a negative standard free energy change. Its equilibrium constant must therefore be what?
     • Less than one
     • Exactly equal to one
     • Negative in value
     • Greater than one

     Correct answer: Greater than one

     Greater than one is the correct answer. A negative standard free energy change favors products at equilibrium, which corresponds to an equilibrium constant greater than one.

236. When solid water sublimes directly to water vapor, the entropy of the substance changes in which way?
     • It increases
     • It decreases
     • It remains unchanged
     • It drops to zero

     Correct answer: It increases

     It increases is the correct answer. Sublimation converts a highly ordered solid into a widely dispersed gas, greatly increasing the number of microstates and the entropy.

237. Dissolving a soluble ionic solid in water typically increases the entropy of the system because the ions do what?
     • Form a rigid lattice
     • Become dispersed throughout the solution
     • Lose all motion
     • Combine into larger crystals

     Correct answer: Become dispersed throughout the solution

     Become dispersed throughout the solution is the correct answer. Dissolution breaks the ordered crystal and spreads ions through the solvent, increasing the dispersal of matter and the entropy.

238. A reaction that is thermodynamically favorable may still proceed extremely slowly because favorability does not determine what?
     • The sign of the free energy change
     • The position of equilibrium
     • The reaction rate
     • The value of the equilibrium constant

     Correct answer: The reaction rate

     The reaction rate is the correct answer. Thermodynamic favorability indicates whether a reaction can occur, but kinetics, not free energy, governs how fast it actually proceeds.

239. In a galvanic cell, as the cell discharges, the mass of the anode generally does what?
     • Increases steadily
     • Stays exactly constant
     • Doubles immediately
     • Decreases over time

     Correct answer: Decreases over time

     Decreases over time is the correct answer. Oxidation at the anode converts metal atoms into ions that enter solution, so the anode loses mass as the cell discharges.

240. When the reaction quotient of a cell equals its equilibrium constant, the Nernst equation predicts the cell potential equals which value?
     • Zero
     • The standard cell potential
     • Faraday's constant
     • The enthalpy change

     Correct answer: Zero

     Zero is the correct answer. When the reaction quotient equals the equilibrium constant the system is at equilibrium, and the Nernst equation yields a cell potential of zero.

241. For a process to be thermodynamically favorable when both enthalpy and entropy changes are positive, the temperature must be what?
     • As low as possible
     • Sufficiently high
     • Exactly absolute zero
     • Held constant at the freezing point

     Correct answer: Sufficiently high

     Sufficiently high is the correct answer. With a positive enthalpy change opposing favorability, a high enough temperature makes the positive temperature-entropy term dominate, giving a negative free energy change.