8.6 Molecular Structure of Acids and Bases

1. A2
   A student who thinks every H atom bonded to oxygen is equally acidic picks this, counting both O–H groups. Only the O–H of the –COOH group ionizes measurably; the O–H on the middle carbon has no neighboring C=O to share the charge of the anion that would form.
2. B1 Correct
   Only the H atom of the –COOH group is acidic. Losing it leaves an anion whose negative charge is shared by two O atoms. The four H atoms bonded to carbon are not donated, and the O–H group on the middle carbon is an alcohol group, which is not ionized to a measurable extent in water.
3. C6
   A student who thinks every H atom in the formula of an acid is acidic picks this, counting all six. The four H atoms bonded to carbon and the alcohol H are not donated to water to a measurable extent.
4. D3
   A student who thinks the acidic hydrogen atoms are the ones written first in a formula picks this, counting the three H atoms on the first carbon atom. Those H atoms are bonded to carbon and are not donated to water; only the H atom of the –COOH group is acidic.

Working No calculation. The molecule has six H atoms: four bonded to C (three on the first C atom, one on the middle C atom), one in the alcohol O–H group on the middle C atom and one in the –COOH group. Only the –COOH hydrogen is donated to water to a measurable extent, so the number is 1.

CED 8.6.A.1 · Read this in Fix

1. A⁻CH₂COOH
   A student who thinks the acidic hydrogen is the one written first in the formula picks this, removing H⁺ from the CH₃ group. The H atoms bonded to carbon are not acidic; the H of the –COOH group is.
2. BCH₃CO⁺
   A student who thinks a compound containing an O–H group releases OH⁻ picks this, the ion left if OH⁻ came off. The O–H group of –COOH is covalently bonded and donates H⁺; it is not released as OH⁻.
3. CCH₃COOH₂⁺
   A student who thinks a conjugate base is formed when an acid gains a proton picks this. The conjugate base is what remains after the acid has lost H⁺, so its charge is one lower than that of the acid, not one higher.
4. DCH₃COO⁻ Correct
   The proton that is transferred is the one bonded to oxygen in the –COOH group. Removing H⁺ from that group leaves CH₃COO⁻, in which the negative charge is shared by the two O atoms.

Working No calculation. The acidic H is the one bonded to O in the –COOH group. Removing H⁺ from it leaves CH₃COO⁻: CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq).

CED 8.6.A.1 · Read this in Fix

1. AEach ion switches rapidly among three forms with one N=O bond each
   A student who thinks an ion flips between its resonance structures picks this. The double-headed arrows do not show a process: the ion has one unchanging structure with three identical N–O bonds.
2. BEach ion keeps its negative charge on the O atom that lost the H⁺ ion
   A student who thinks the charge of an oxyanion stays on the O atom that was bonded to H picks this. The three O atoms of NO₃⁻ are equivalent, and the charge is shared equally among them.
3. CEach ion spreads its charge evenly and has N–O bonds that are alike Correct
   The three diagrams are equivalent drawings of a single structure. In the real ion the three N–O bonds are identical and the negative charge is shared equally by all three O atoms. Spreading the charge in this way stabilizes NO₃⁻, which is why it is a very weak base.
4. DEach ion has one of three fixed structures, present in equal numbers
   A student who thinks each resonance structure is a different particle in a mixture picks this. All NO₃⁻ ions are identical, and none has the structure shown by a single diagram.

Working No calculation. Resonance structures are alternative drawings of one structure. The real NO₃⁻ ion has three identical N–O bonds, and its negative charge is shared equally by the three O atoms; this delocalization stabilizes the ion.

CED 8.6.A.1.i · Read this in Fix

1. AEach added O atom pulls electron density toward itself, which spreads out the charge of the conjugate base Correct
   Oxygen is highly electronegative. Each additional O atom bonded to Cl draws electron density away from the O–H end of the molecule and gives the conjugate base another O atom to share its negative charge. The more stable the conjugate base, the stronger the acid, which matches the trend from HClO to HClO₄.
2. BEach added O atom raises the molar mass of the molecule, which lets the heavier acid release its H⁺ more readily
   A student who thinks heavier acid molecules are stronger acids picks this. Molar mass does rise along the series, but mass does not cause ionization; the extra O atoms matter because they stabilize the conjugate base.
3. CEach added O atom strengthens the O–H bond of the molecule, which makes the acid stronger as the bond gets stronger
   A student who thinks a strong acid is one with a strong bond to H picks this. 'Strong' refers to the extent of ionization; the trend is explained by the growing stability of the conjugate base, not by a stronger O–H bond.
4. DEach added O atom makes the conjugate base a stronger base, which is why the acid it comes from is stronger too
   A student who thinks a strong acid has a strong conjugate base picks this. The relationship is the reverse: the extra O atoms make the conjugate base more stable and therefore a weaker base, and that is why the acid is stronger.

Working No calculation. The table shows acid strength increasing with the number of O atoms bonded to Cl (K_a 3 × 10⁻⁸ → 1 × 10⁻² → strong). Each extra electronegative O atom withdraws electron density and gives the conjugate base more O atoms over which its negative charge is spread, so the conjugate base is more stable and the acid is stronger.

CED 8.6.A.1.i · Read this in Fix

1. AClose to 13, because the conjugate base of a strong acid is itself a strong base
   A student who thinks a strong acid has a strong conjugate base picks this, treating 0.10 M Br⁻ like 0.10 M OH⁻. The conjugate base of a strong acid is a very weak base, so Br⁻ does not produce OH⁻.
2. BEqual to 7, because Br⁻ is too weak a base to remove H⁺ ions from water molecules Correct
   Br⁻ is the conjugate base of the strong acid HBr. It is so stable that it has a negligible tendency to accept a proton, so it does not react with water. With neither ion reacting, the solution is neutral: pH 7 at 25°C.
3. CSlightly above 7, because an ion with a negative charge pulls H⁺ ions out of water
   A student who thinks every negative ion acts as a base picks this. Charge alone does not make an anion basic: Br⁻ is the conjugate base of a strong acid and does not remove H⁺ from water to a measurable extent.
4. DSlightly below 7, because Br⁻ comes from the acid HBr and keeps some of its acidity
   A student who thinks the anion of an acid stays acidic picks this. Br⁻ has no proton to donate; it is the conjugate base of HBr, and it is too weak a base to change the pH.

Working No calculation. Br⁻ is the conjugate base of a strong acid, so it is a very weak base that does not accept H⁺ from water to a measurable extent. Neither ion reacts with water, so [H₃O⁺] = [OH⁻] and the pH is 7 at 25°C.

CED 8.6.A.1.i · Read this in Fix

1. ASolution 1 Correct
   Propanoic acid is a carboxylic acid, a weak acid. Its solution is acidic, but only a small percentage of the molecules are ionized, so [H₃O⁺] is far below 0.10 M and the pH is well above 1.00. The solution with pH 2.94 is the only one that fits; pH 1.00 belongs to the strong acid HNO₃.
2. BSolution 2
   A student who thinks every acid ionizes completely picks the solution with pH 1.00, expecting [H₃O⁺] = 0.10 M. That solution is the strong acid HNO₃; a carboxylic acid is only slightly ionized, so its pH is higher.
3. CSolution 3
   A student who thinks a weak acid hardly reacts and leaves water neutral picks the solution with pH 7.00. That solution is ethanol, which is not ionized measurably; a weak acid ionizes enough to bring the pH of a 0.10 M solution to about 3.
4. DSolution 4
   A student who thinks a compound with OH in its formula is a base picks the solution with pH 13.00. That solution is KOH, an ionic hydroxide; the O–H of a –COOH group is covalently bonded and donates H⁺, so propanoic acid is an acid.

Working No calculation is needed. HNO₃ is a strong acid: 0.10 M gives pH 1.00. KOH is a strong base: 0.10 M gives pH 13.00. Ethanol is not ionized measurably: pH 7.00. Propanoic acid is a carboxylic acid, a weak acid: acidic, but with [H₃O⁺] far below 0.10 M, so its pH lies between 1 and 7: the solution with pH 2.94. (Check: K_a = 1.3 × 10⁻⁵, [H₃O⁺] ≈ √(1.3 × 10⁻⁵ × 0.10) = 1.1 × 10⁻³ M, pH 2.94.)

CED 8.6.A.1.ii · Read this in Fix

1. ACH₃CH₂CH₂OH
   A student who thinks every O–H hydrogen is equally acidic picks this alcohol. Without a neighboring C=O to share the charge, the anion that would form is not stabilized, and an alcohol is not ionized measurably in water.
2. BCH₃COOCH₃
   A student who reads 'COO' in a formula as a carboxylic acid picks this. Here the second O atom is bonded to a CH₃ group, not to H, so there is no O–H hydrogen to donate.
3. CCH₃CH₂CH₂NH₂
   A student who thinks a substance with H atoms and no OH group donates H⁺ as an acid does picks this amine. Its N atom uses a lone pair to accept a proton from water, so it is a weak base, not an acid.
4. DCH₃CH₂COOH Correct
   This molecule contains the –COOH group, so it is a carboxylic acid, a common class of weak acid. The H atom bonded to O in –COOH is donated to water to a small extent, leaving an anion whose charge is shared by two O atoms.

Working No calculation. A weak acid of this class needs a –COOH group. CH₃CH₂COOH has one. CH₃CH₂CH₂OH has only an alcohol O–H; CH₃COOCH₃ has no O–H at all (the second O atom is bonded to CH₃); CH₃CH₂CH₂NH₂ is an amine, a nitrogenous weak base.

CED 8.6.A.1.ii · Read this in Fix

1. AIt is fully dissociated into Ca²⁺ and OH⁻ ions, but Ca(OH)₂ is a weak base because the [OH⁻] it gives is low
   A student who equates strength with concentration picks this. A low [OH⁻] here reflects low solubility; because all of the dissolved Ca(OH)₂ is dissociated, it is a strong base.
2. BIt is mostly undissociated Ca(OH)₂ units, so Ca(OH)₂ is a weak base, as its low solubility would suggest
   A student who thinks 'slightly soluble' means 'slightly dissociated' picks this. Solubility limits how much dissolves; the part that does dissolve is completely dissociated into ions.
3. CIt is fully dissociated into Ca²⁺ and OH⁻ ions, so Ca(OH)₂ is a strong base even though little of it dissolves Correct
   Group 2 hydroxides are strong bases. Whatever amount of Ca(OH)₂ dissolves is completely dissociated into Ca²⁺ and OH⁻ ions. Strength refers to that complete dissociation, not to how much of the solid dissolves.
4. DIt is mostly undissociated Ca(OH)₂ units, but Ca(OH)₂ is a strong base because it has two OH groups
   A student who thinks more OH groups in a formula make a stronger base picks this. Strength is the extent of dissociation, which is complete for dissolved Ca(OH)₂; it is not set by the number of OH groups.

Working No calculation. Ca(OH)₂ is a group 2 hydroxide, a strong base: the portion that dissolves is completely dissociated, Ca(OH)₂(s) → Ca²⁺(aq) + 2OH⁻(aq). Low solubility limits how much dissolves, not the fraction of the dissolved base that is dissociated.

CED 8.6.A.1.iii · Read this in Fix

1. AH₂O, which is a very weak acid Correct
   Adding a proton to OH⁻ gives H₂O, so H₂O is its conjugate acid. Because OH⁻ is a strong base that holds an accepted proton firmly, its conjugate acid has very little tendency to donate that proton: H₂O is a very weak acid.
2. BH₂O, which is a strong acid
   A student who thinks a strong base has a strong conjugate acid picks this. The species is right, but the relationship is inverse: the conjugate acid of a strong base is a very weak acid.
3. CK⁺, which is a very weak acid
   A student who pairs the two ions of KOH as a conjugate pair picks this. The base is OH⁻, and its conjugate acid is OH⁻ plus one proton, H₂O; K⁺ is a spectator ion.
4. DH₃O⁺, which is a strong acid
   A student who thinks H₃O⁺ is the conjugate acid of every base in water picks this. H₃O⁺ is the conjugate acid of H₂O; adding one proton to OH⁻ gives H₂O.

Working No calculation. OH⁻ + H⁺ → H₂O, so the conjugate acid of OH⁻ is H₂O. The conjugate acid of a strong base is a very weak acid; K⁺ is a spectator ion.

CED 8.6.A.1.iii · Read this in Fix

1. ADiagram 1
   A student who thinks a weak base does not react with water picks the diagram that shows only NH₃ molecules. A small fraction of the molecules do accept a proton, so the solution contains some NH₄⁺ and OH⁻ ions.
2. BDiagram 2 Correct
   Ammonia is a weak base. Most of the dissolved NH₃ remains as molecules, and each of the few that accept a proton from water forms one NH₄⁺ ion and leaves one OH⁻ ion. The diagram with five NH₃ molecules, one NH₄⁺ ion and one OH⁻ ion shows this.
3. CDiagram 3
   A student who thinks every base reacts completely picks the diagram that shows only NH₄⁺ and OH⁻ ions. That would describe a strong base; in an ammonia solution most of the NH₃ remains as molecules.
4. DDiagram 4
   A student who thinks a substance with H atoms and no OH group donates H⁺ picks the diagram that shows NH₂⁻ and H₃O⁺ ions. In water, NH₃ accepts a proton with the lone pair on its N atom, forming NH₄⁺ and leaving OH⁻.

Working No calculation. NH₃ is a weak base: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) lies far to the left. Most NH₃ stays as molecules, and each molecule that reacts gives one NH₄⁺ and one OH⁻. The diagram with five NH₃, one NH₄⁺ and one OH⁻ fits.

CED 8.6.A.1.iv · Read this in Fix

1. AThe CH₃COO⁻ ion is a strong base, so nearly all of these ions accept H⁺ from water, leaving OH⁻
   A student who thinks the conjugate base of a weak acid is a strong base picks this. Acetate is a weak base: only a small fraction of the ions react with water, which is why the solution is only slightly basic. If nearly all of the acetate ions reacted, [OH⁻] would be about 0.10 M and the pH about 13, not 8.9.
2. BThe Na⁺ ion reacts with water to re-form NaOH, so this strong base then releases OH⁻ into the solution
   A student who thinks the cation of a strong base regenerates that base in water picks this. Na⁺ does not react with water; the OH⁻ comes from water molecules that have given a proton to CH₃COO⁻.
3. CThe CH₃COO⁻ ion is a weak base, so a small fraction of these ions accept H⁺ from water, leaving OH⁻ Correct
   Carboxylate ions are weak bases. A small fraction of the CH₃COO⁻ ions accept a proton from water molecules, forming CH₃COOH and leaving OH⁻ ions, so [OH⁻] rises above [H₃O⁺]. Na⁺ takes no part.
4. DThe CH₃COO⁻ ion is a base, so it releases OH⁻ ions made from the O and H atoms of its own formula
   A student who thinks a base must supply OH⁻ from its own formula picks this. Acetate stays intact and accepts a proton; the OH⁻ ions are what is left of the water molecules that donated the protons.

Working No calculation. CH₃COO⁻ is the conjugate base of the weak acid CH₃COOH, so it is a weak base: CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq), with only a small fraction of the ions reacting. Na⁺ does not react with water.

CED 8.6.A.1.iv · Read this in Fix

1. AIt is about 4 times as great
   A student who reads a difference in pK_a as a simple ratio picks this. The scale is logarithmic: a difference of about 4 units corresponds to a factor of about 10⁴ in K_a.
2. BIt is about 40 times as great
   A student who counts a factor of ten for each pK_a unit but adds the factors (10 × 4) picks this. The factors multiply: 10 × 10 × 10 × 10 = 10⁴.
3. CIt is about 10⁻⁴ times as great
   A student who thinks a larger pK_a means a stronger acid picks this, taking acetic acid to have the larger K_a. A lower pK_a means a larger K_a, so the acid with three Cl atoms has the larger K_a.
4. DIt is about 10⁴ times as great Correct
   The points are at pK_a ≈ 4.8 (no Cl) and pK_a ≈ 0.7 (three Cl), a difference of about 4 units. Because pK_a = −log K_a, each unit is a factor of 10 in K_a and a lower pK_a means a larger K_a, so K_a is about 10⁴ times as great.

Working From the graph, pK_a ≈ 4.8 for 0 Cl atoms and pK_a ≈ 0.7 for 3 Cl atoms: a difference of about 4 units. pK_a = −log K_a, so K_a(3 Cl)/K_a(0 Cl) = 10^{4.8 − 0.7} ≈ 10⁴. The lower pK_a belongs to the larger K_a, so the K_a of the acid with three Cl atoms is about 10⁴ times as great.

CED 8.6.A.1.v · Read this in Fix

1. AHClO, because the H–Cl bond in HClO is more polar than the H–I bond in the molecule HIO
   A student who thinks the H atom of an oxyacid is bonded to the halogen picks this. Neither molecule has a bond between H and the halogen: the H atom is bonded to O in both.
2. BHIO, because acid strength increases down group 17, just as it does from HCl to HBr to HI
   A student who extends the trend for the binary acids to oxyacids picks this. In HCl, HBr and HI the H is bonded directly to the halogen; in HClO and HIO it is bonded to O, and the more electronegative halogen gives the stronger acid.
3. CHClO, because Cl is more electronegative than I and stabilizes the conjugate base more Correct
   In both acids the H atom is bonded to O, which is bonded to the halogen. The more electronegative Cl atom pulls more electron density away from the O atom, so the negative charge of ClO⁻ is better stabilized than that of IO⁻, and HClO ionizes to the greater extent.
4. DHIO, because the less electronegative I atom holds the electrons, and so the H atom, less tightly
   A student who thinks an electronegative atom holds the acidic H more tightly picks this. The electronegative atom stabilizes the anion left when H⁺ leaves, so the acid with the more electronegative halogen, HClO, is stronger.

Working No calculation. Both are oxyacids, H–O–X. Cl is more electronegative than I, so it withdraws more electron density from the O atom and stabilizes the conjugate base OX⁻ more. HClO is the stronger acid (K_a about 3 × 10⁻⁸, compared with about 2 × 10⁻¹¹ for HIO).

CED 8.6.A.1.v · Read this in Fix

1. AIt is lower, because the Cl atom helps stabilize the negative charge of the conjugate base ion Correct
   Chlorine is electronegative and pulls electron density toward itself, spreading out the negative charge of ClCH₂COO⁻. The more stable conjugate base makes ClCH₂COOH the stronger acid, so at the same concentration its solution has the higher [H₃O⁺] and the lower pH.
2. BIt is higher, because the Cl atom attracts electrons and holds the acidic H atom more tightly
   A student who thinks an electronegative atom holds the acidic H more tightly picks this. The Cl atom stabilizes the anion that is left when H⁺ leaves, which makes the acid stronger and the pH lower.
3. CIt is the same, because both acids donate H⁺ from an identical –COOH group at equal concentration
   A student who thinks acid strength is fixed by the acidic group alone picks this. The Cl atom near the –COOH group stabilizes the conjugate base, so the two acids ionize to different extents.
4. DIt is higher, because ClCH₂COOH has fewer H atoms in its formula to donate than CH₃COOH has
   A student who thinks more H atoms make a stronger acid picks this. Each acid donates only the H of its –COOH group; the H atoms bonded to carbon do not affect how many protons are donated.

Working No calculation is needed. The electronegative Cl atom withdraws electron density and stabilizes the conjugate base ClCH₂COO⁻ relative to the acid, so ClCH₂COOH is the stronger acid (K_a = 1.4 × 10⁻³ compared with 1.8 × 10⁻⁵). At equal concentration it gives the higher [H₃O⁺] and the lower pH.

CED 8.6.A.1.v · Read this in Fix

1. AThe closer the Cl atom is to the –COOH group, the more it lowers the strength of the resulting acid
   A student who thinks a larger pK_a means a stronger acid picks this, reading 2.86 as the weakest acid. pK_a = −log K_a, so the lowest pK_a belongs to the strongest acid: Cl makes the acid stronger, most of all when it is close.
2. BThe closer the Cl atom is to the –COOH group, the more it stabilizes the charge of the conjugate base Correct
   A lower pK_a means a stronger acid, so the acid with Cl on C2 is the strongest and the effect shrinks as Cl moves to C3 and C4. The electronegative Cl atom withdraws electron density through the bonds, and this stabilizes the negative charge of the carboxylate ion most when the Cl atom is nearest to it.
3. CThe closer the Cl atom is to the –COOH group, the more it strengthens the O–H bond in that group
   A student who thinks a strong acid is one with a strong bond to H picks this. Acid strength is the extent of ionization; the pattern is explained by the stability of the conjugate base, not by a stronger O–H bond.
4. DThe closer the Cl atom is to the –COOH group, the more it raises the base strength of the anion
   A student who thinks a stronger acid has a stronger conjugate base picks this. A nearby Cl atom makes the anion more stable and so a weaker base, and that is why the acid is stronger.

Working No calculation. Lower pK_a means stronger acid. Order of strength: Cl on C2 (2.86) > Cl on C3 (4.05) > Cl on C4 (4.52) > no Cl (4.82). The Cl atom withdraws electron density through the bonds; the effect stabilizes the carboxylate ion most when Cl is closest to the –COOH group and fades with distance.

CED 8.6.A.1.v · Read this in Fix