CHAPTER 11:

AClDS AND BASES

The following information is often presented on the first page of exams covering this chapter under the heading: “potentially useful information.”

[X] = concentration of X at equilibrium, in moles per liter

(X) = concentration of X at any moment in time, in moles per liter

C_X = initial concentration of X, in moles per liter

Acids, Bases, and Salts

11-1. Which of the following groups contains salts that all form basic solutions in water?

(a) NaNO₃, NH₄CN, NaOAc, NH₄Cl

(b) Na₂CO₃, KCl, NaOAc, NH₄Cl

(d) NaHCO₃, NaF, NH₄Cl, Na₂SO₃

(e) none of the above

Answer: (c)

11-2. Which of the following compounds would give an 0.10 M solution with a pH less than 7?

(a) Na₂S (b) K₂CO₃ (c) KCN (d) NH₄Cl (e) Na₃PO₄

Answer: (d)

11-3. Which of the following is a strong acid?

(a) HNO₃ (b) H₂S (c) HNO₂ (d) HCO₃^- (e) HOCl

Answer: (a)

11-4. Which of the following is a strong electrolyte?

(a) Ca(CN)₂ (b) NH₃ (c) HCN (d) H₂S (e) acetic acid

Answer: (a)

11-5. Which of the following compounds is not an Arrhenius acid?

(a) HCl (b) H₂SO₄ (c) H₂O (d) FeCl₃ (e) H₂S

Answer: (d)

11-6. Which of the following compounds is not an Arrhenius base?

(a) NaOH (b) MgO (c) Ca(OH)₂ (d) H₂O (e) NH₃

Answer: (e)

11-7. The following compounds dissolve in water to form solutions that turn litmus from red to blue. Which of these compounds satisfy the operational definition of a base?

(a) NaHCO₃ (b) CaO (c) CaH₂ (d) NH₃ (e) all of the above

Answer: (e)

11-8. What is the H₃O⁺ ion concentration in a solution that has a pH of 5.75?

(a) 5.8 x 10^-5 (b) 3.2 x 10^-3 (c) 0.75 x 10^-5 (d) 1.8 x 10^-6

Answer: (d)

11-9. Element X reacts with excess oxygen to form a basic oxide with the formula XO. In which Group of the periodic table should X be placed?

(a) Group IIA (b) Group IIIA (c) Group IVA (d) Group VIA (e) Group VIIA

Answer: (a)

11-10. If the element described in the previous question were to react with hydrogen it should form:

(a) an acidic hydride, XH (b) an acidic hydride, XH₂

(e) none of the above

Answer: (d)

11-11. Which Group IVa element is most likely to form an oxide with the formula XO₂ that dissolves in water to form an acidic solution?

(a) C (b) Si (c) Ge (d) Sn (e) Pb

Answer: (a)

11-12. Which of the following elements would be the most likely to form an acidic oxide with the formula XO₂ and an acidic hydride with the formula XH₂?

(a) Na (b) Mg (c) Al (d) S (e) Cl

Answer: (d)

11-13. Which compound should dissolve in water to form an 0.1 M solution with a pH of 8?

(a) HCl (b) NaOAc (CH₃CO₂Na) (c) HOAc (CH₃CO₂H)

(d) NaCl (e) KOH

Answer: (b)

11-14. Which of the following compounds exhibits the following behavior. It dissolves in water to give a colorless, odorless solution with a pH of 11. This solution reacts with HCl to form an odorless gas. When AgNO₃ is added to this solution, a yellow precipitate is formed.

(a) H₂SO₄ (b) Cu(OH)₂ (c) Na₂CO₃ (d) Na₂S (e) NaCl

Answer: (c)

11-15. Which of the following acid-base reactions should go more or less to completion?

(a) HClO₄(aq) + OCl^-(aq) ➝ ClO₄^-(aq) + HOCl(aq)

(b) HF(aq) + I^-(aq) ➝ HI(aq) + F^-(aq)

(d) H₂S(aq) + 2 Cl^-(aq) ➝ S^2-(aq) + 2 HCl(aq)

(e) All of these reactions would go to completion, giving high yields of products.

Answer: (a)

11-16. Which of the following won’t produce an acidic solution when dissolved in water? (a) FeCl₃ (b) NH₄Cl (c) acetic acid (CH₃CO₂H)

(d) sodium acetate (CH₃CO₂Na) (e) HClO₄

Answer: (d)

11-17. Which of the following statements is correct?

(a) nonmetals usually form basic oxides

(b) metals usually form basic hydroxides

(d) nonmetals usually form ionic compounds with other nonmetals

(e) all of the above are correct.

Answer: (b)

11-18. Which of the following gives a basic solution when dissolved in water?

(a) CaO (b) CO₂ (c) Cl₂O (d) SO₂ (e) SeO₃

Answer: (a)

11-19. Which of the following compounds isn’t an acid in water?

(I) SO₂ (II) HNO₃ (III) SrO (IV) HI (V) K₂S

(a) I and II (b) II and III (c) III and V

(d) I, II and IV (e) all of the above are acids in water

Answer: (c)

11-20. Which of the following compounds isn’t a base in water?

(a) Ca(OH)₂ (b) CuO (c) NO₂ (d) Li₂O (e) CsOH

Answer: (c)

11-21. Which of the following is the most likely to be a white solid with a high melting point that dissolves in water to form a basic solution?

(a) O₂ (b) CO₂ (c) Na₂O (d) P₄O₁₀ (e) Cl₂O₇

Answer: (c)

11-22. Which of the following oxides would yield the most basic solution when dissolved in water?

(a) ClO₂ (b) SiO₂ (c) SO₂ (d) Al₂O₃

Answer: (d)

11-23. Which of the following isn’t an acid-base reaction.

(a) 2 NaCl(s) + MnO₂(s) + 4 H⁺(aq) ➝ 2 Na⁺(aq) + Mn²⁺(aq) + H₂O(l) + Cl₂(g)

(b) 2 NaCl(s) + H₂SO₄(aq) ➝ Na₂SO₄(aq) + 2 HCl(g)

(d) H₂C₂O₄(aq) + 2 NaOH(aq) ➝ Na₂C₂O₄(aq) + 2 H₂O(l)

(e) all of the above are acid-base reactions

Answer: (a)

Brønsted Acids and Bases

11-24. Consider the following reaction:

H₂PO₄^-(aq) + HCO₃^-(aq) ➝ H₂CO₃(aq) + HPO₄^2-(aq)

Brønsted would identify the acidic species as:

(a) H₂PO₄^- and H₂CO₃ (b) H₂PO₄^- and HPO₄^2-

(e) H₂CO₃ and HPO₄^2-

Answer: (a)

11-25. Label the Brønsted acids and bases in the following reaction.

HSO₄^-(aq) + H₂O(l) ➝ H₃O⁺(aq) + SO₄^2-(aq)

Answer: acids = HSO₄^- and H₃O⁺; bases = H₂O and SO₄^2-

11-26. Which compound cannot act as a Brønsted base?

(a) NH₃ (b) H₂O (c) CH₄ (d) Cl^- (e) H₃O⁺

Answer: (c)

11-27. Which of the following compounds cannot be an base?

(a) H₂O (b) NH₃ (c) CO₃^2- (d) OH^- (e) NH₄⁺

Answer: (e)

11-28. Which of the following compounds cannot be a Brønsted base?

(a) OH^- (b) H₂O (c) NH₃ (d) NH₄⁺

(e) all of the above are Brønsted bases

(e) all of these compounds can be Brønsted bases.

Answer: (d)

11-29. Which of the following compounds won’t be a base in solution?

(a) NH₃ (b) CaO (c) Ca(OH)₂ (d) NaOH (e) NH₄NO₃

Answer: (e)

11-30. Which of the following isn’t a Brønsted base?

(a) PF₅ (b) H₂O (c) NH₃ (d) S^2- (e) CN^-

Answer: (a)

11-31. Which of the following compounds cannot be a Brønsted base?

(a) H₂O (b) MnO₄^- (c) BH₄^- (d) CN^- (e) S^2-

Answer: (c)

11-32. Which of the following compounds cannot be a Brønsted base?

(a) O₂ (b) CO₂ (c) PH₃ (d) SF₄ (e) CH₃⁺

Answer: (e)

11-33. Which of the following compounds can act as either a Brønsted acid or a Brønsted base?

(I) NaHCO₃ (II) Na₂CO₃ (III) H₂CO₃ (IV) CO₂ (V) H₂O

(a) I (b) I and II (c) II and III (d) I and V (e) V

Answer: (d)

11-34. Which compound is the most likely to be amphoteric?

(a) Na₂O (b) CaO (c) Al₂O₃ (d) P₄O₁₀ (e) Cl₂O₇

Answer: (c)

11-35. Predict the products of the following acid-base reaction.

NaOCH₃(aq) + NaHCO₃(aq) ➝

Answer: CH₃OH and CO₃^2-

11-36. Predict the products of the following acid-base reaction.

HOCl(aq) + CH₃NH₂(aq) ➝

Answer: CH₃NH₃⁺ and OCl^-

11-37. Predict the products of the following acid-base reaction.

NH₄Cl(aq) + NaSH(aq) ➝

Answer: H₂S and NH₃

Conjugate Acid/Base Pairs

11-38. Which of the following is the conjugate acid of the HPO₄^2- ion?

(a) H₃PO₄ (b) H₂PO₄^- (c) HPO₄^2- (d) PO₄^3- (e) H₃O⁺

Answer: (b)

11-39. What is the conjugate base of the HPO₄^2- ion?

(a) PO₄^3- (b) HPO₄^2- (c) H₂PO₄^- (d) H₃PO₄ (e) none of the above

Answer: (a)

11-40. What is the conjugate base of HSO₄^-?

(a) H₂O (b) H₂SO₄ (c) HSO₄^- (d) SO₃ (e) none of the above

Answer: (e)

11-41. Methanol is described by the following skeleton structure.

Which compound is formed when CH₃OH acts as a Brønsted base?

(a) CH₃O^- (b) CH₃OH (c) CH₃OH₂⁺ (d) CH₃OH (e) H₃O⁺

Answer: (c)

11-42. Which of the following is the conjugate base of a strong acid?

(a) OH^- (b) HSO₄^- (c) NH₂^- (d) S^2- (e) H₃O⁺

Answer: (b)

11-43. “Cocaine” as it is shown on television is actually a salt known as cocaine hydrochloride, [C₁₇H₂₂NO₄⁺][Cl^-], which forms white crystals that stupid people snort through straws to make them even more stupid. Recently, a lot of attention has been paid to a frightening substance known as “crack”, which is formed by treating cocaine with sodium bicarbonate, NaHCO₃. “Crack” is a neutral compound with the formula C₁₇H₂₁NO₄. Which of the following statements is true?

(a) “Crack” is the conjugate base of cocaine hydrochloride.

(b) “Cocaine” contains the conjugate acid of “crack”.

(d) “Crack” can be “smoked” because it is a covalent compound that becomes reasonably volatile when heated, but “cocaine” must be snorted because it is a salt that does not easily boil even when heated.

(e) All of the above are true.

Answer: (e)

11-44. Which of the following represents a conjugate acid/base pair?

(a) H⁺, OH^- (b) H₃O⁺, OH^- (c) NH₃, NH₄⁺ (d) NaOH, HCl

(e) all of the above are conjugate acid/base pairs

Answer: (c)

11-45. Which of the following isn’t a conjugate acid/base pair?

(a) NH₄⁺(aq) and NH₃(aq) (b) H₂O(l) and OH^-(aq)

(e) all of these are conjugate acid/base pairs.

Answer: (c)

11-46. Which of the following isn’t a Brønsted conjugate acid/base pair?

(a) NH₃/NH₂^- (b) CH₃CO₂H/CH₃CO₂^- (c) H₂O/O^2- (d) H₃O⁺/H₂O

Answer: (c)

Relative Strength of Acids and Bases

11-47. Which of the following is the strongest Brønsted acid?

(a) NH₂^- (b) HS^- (c) H₂O (d) CH₄ (e) NH₄⁺

Answer: (e)

11-48. Which of the following is the strongest Brønsted acid?

(a) NH₄ (b) HCl (c) HNO₃ (d) HSO₄^- (e) HClO₄

Answer: (e)

11-49. Which of the following is the strongest Brønsted acid?

(a) CsH (b) H₂O (c) HF (d) H₂Te (e) HI

Answer: (e)

11-50. Which of the following would be the strongest Brønsted acid?

(a) H₃O⁺ (b) HF (c) NH₃ (d) NaHSO₄ (e) NaOH

Answer: (a)

11-51. Which of the following would be the weakest Brønsted acid?

(a) H₂Se (b) H₂S (c) H₂O (d) SH^- (e) OH^-

Answer: (e)

11-52. Which of the following would be the weakest Brønsted acid?

(a) HClO₄ (b) H₃PO₄ (c) H₂CO₃ (d) H₂SiO₄ (e) Al(OH)₃

Answer: (e)

11-53. Which of the following is the weakest acid?

(a) HClO₄ (b) HCl (c) HF (d) HI (e) HBr

Answer: (c)

11-54. Which of the following is the weakest Brønsted acid?

(a) H₂SO₃, K_a = 0.3 (b) H₂CrO₄, K_a = 9.6

Answer: (d)

11-55. Which of the following is the strongest base?

(a) I^- (b) NO₃^- (c) HS^- (d) O^2- (e) OH^-

Answer: (d)

11-56. Which of the following is the strongest base?

(a) NH₂^- (b) PH₂^- (c) CH₃^- (d) SiH₃^- (e) OH^-

Answer: (c)

11-57. Which of the following is the strongest base?

(a) H₂O (b) OH^- (c) NH₂^- (d) NH₃ (e) CH₄

Answer: (c)

11-58. Of the following, which is the strongest base?

(a) F^- (b) OH^- (c) H₂O (d) Cl^- (e) O^2-

Answer: (e)

11-59. Which of the following would be the strongest Brønsted base?

(a) HSO₃^- (b) HSO₄^- (c) H₂SO₃ (d) SO₃^2- (e) SO₄^2-

Answer: (d)

11-60. Which of the following would be the strongest Brønsted base?

(a) H₂O (b) SH^- (c) OH^- (d) S^2- (e) O^2-

Answer: (e)

11-61. Which of the following would be the strongest Brønsted base?

(a) InH₂^- (b) SnH₃^- (c) SbH₂^- (d) The^- (e) I^-

Answer: (b)

11-62. Which oxide would dissolve in water to produce the most basic solution?

(a) CaO (b) Ga₂O₃ (c) GeO₂ (d) P₄O₁₀ (e) SO₃

Answer: (a)

11-63. Which of the following industrial compounds is the strongest base?

(a) NH₄NO₃ (b) (NH₄)₂SO₄ (c) Na₂CO₃ (d) CO₂

Answer: (c)

11-64. Which of the following is a weak base?

(a) (CH₃)₃N (b) HF (c) KOH (d) Na (e) H₂

Answer: (a)

11-65. Which of the following would form the most basic 0.100 M solution?

(a) NaHSO₄ (b) NaHCO₃ (c) NaH₂PO₄ (d) Na₂CO₃ (e) NaCl

Answer: (d)

11-66. Which of the following would have the strongest conjugate base?

(a) H₂O (b) H₂S (c) NH₃ (d) PH₃ (e) CH₄

Answer: (e)

11-67. Arrange the following bases in order of increasing strength: NH₃, PH₃, H₂O and H₂S.

Answer: H₂S < H₂O < PH₃ < NH₃

11-68. Which of the following is correct?

(a) K₂SO₃ is a stronger base than KHSO₃

(b) K₂CO₃ is a weaker base than KHCO₃

(d) Na₂HPO₄ is a weaker base than NaH₂PO₄.

(e) All of these are correct

Answer: (a)

11-69. Which of the following statements is true?

(a) H₂O is a stronger acid than HF(aq)

(b) H₂O is a stronger acid than H₂S(aq)

(d) H₂O is a stronger acid than NH₃(aq)

(e) None of the above are true

Answer: (d)

11-70. Which of the following statements is true?

(a) H₂O is a weaker acid than NH₃

(b) PH₃ is a weaker acid than NH₃

(d) NH₂^- is the conjugate acid of NH₃

(e) ClO^- is a stronger base than ClO₄^-

Answer: (e)

11-71. What can be correctly concluded from the fact that the following the acid-base reaction proceeds to the right, as written?

NH₂^-(aq) + HSO₄^-(aq) ➝ NH₃(aq) ⁺SO₄^2-(aq)

(a) NH₃ is a stronger base than NH₂^-

(b) NH₃ is a stronger acid than HSO₄^-

(d) NH₃ is a weaker base than HSO₄^-

(e) NH₃ is a weaker acid than HSO₄^-

Answer: (e)

11-72. NaHCO₃ can be used to neutralize strong bases, such as NaOH. What conclusion can be drawn from the fact that the following acid-base reaction proceeds to the right as written?

HCO₃^-(aq) + OH^-(aq) ➝ CO₃^2-(aq) + H₂O(l)

(a) HCO₃^- is a stronger acid than H₂O

(b) HCO₃^- is a stronger base than CO₃^2-

(d) CO₃^2- is a stronger base than OH^-

(e) H₂O is a stronger acid than HCO₃^-

Answer: (a)

11-73. What can we conclude from the fact that the following reaction proceeds as written?

NaNH₂(s) + H₂O(l) ➝ NH₃(aq) + OH^-(aq)

(a) OH^- is a stronger base than NH₂^-

(b) NH₃ is a stronger acid than H₂O

(d) OH^- is a weaker base than NH₂^-

(e) More than one of the above is true

Answer: (d)

Lewis Acids and Bases

11-74. Which compound cannot accept a pair of electrons and function as a Lewis acid?

(a) SO₃ (b) BF₃ (c) Ag⁺ (d) CH₄ (e) H⁺

Answer: (d)

General Acid-Base Questions

Use the following acid-dissociation equilibrium constants for questions 75-77.

HA: K_a = 1.00 x 10^-4 HB: K_a = 1.00 x 10^-7

HC: K_a = 1.00 x 10^-10 HD: K_a = 1.00 x 10^-11

11-75. Solutions of each acid are prepared in which the initial concentration of the acid is 0.1000 M. Which of the four solutions will be the most acidic?

(a) HA (b) HB (c) HC (d) HD

(e) not enough information is provided to answer this question.

Answer: (a)

11-76. 0.5 M solutions of NaA, NaB, NaC, and NaD are prepared. Which solution is the most basic?

(a) NaA (b) NaB (c) NaC (d) NaD

(e) all of the solutions have the same pH.

Answer: (d)

11-77. A solution is prepared with initial concentrations of HA and NaA of 0.50 M and 1.00 M respectively. What is the pH of the solution?

(a) 3.70 (b) 4.00 (c) 4.30 (d) 9.70 (e) 10.30

Answer: (c)

11-78. Which of the following solutions would have a pH greater than 7.0?

(a) NH₄Cl (b) NaCl (c) HOAc (d) NaOAc

Answer: (d)

11-79. Which of the following solutions has a pH above 7.0?

(a) 0.10 M NH₄Cl (b) 0.10 M NH₄NO₃ (c) 0.10 M HCN

(d) 0.10 M NaCN (e) none of these

Answer: (d)

11-80. Which of the following negative ions would be the strongest base?

(a) Cl₂CHCO₂^- (Cl₂CHCOC₂H: K_a = 7.8 x 10^-3)

(b) ClCH₂CO₂^- (ClCH₂CO₂H: Ka = 1.4 x 10^-3)

(d) CH₃CO₂^- (CH₃CO₂H: K_a = 1.8 x 10^-5)

Answer: (d)

11-81. Which of the following acids would have the strongest conjugate base?

(a) HOCl (K_a = 2.9 x 10^-8)(b) HOBr (K_a = 2.4 x 10^-9)

(e) H₂O (K_a = 1.8 x 10^-16)

Answer: (e)

11-82. Which of the following solutions would be the most acidic?

(a) 0.10 M CH₃CO₂H (K_a = 1.8 x 10^-5) (b) 0.10 M HCO₂H (K_a = 1.8 x 10^-4)

(e) All of these solutions would have the same pH.

Answer: (d)

11-83. Which of the following is the strongest Brønsted acid?

(a) NH₄⁺ (b) HCl (c) HNO₃ (d) HSO₄^- (e) HClO₄

Answer: (e)

11-84. Which of the following is the strongest base?

(a) I^- (b) NO₃^- (c) HS^- (d) O^2- (e) OH^-

Answer: (d)

11-85. Which of the following is the strongest acid?

(a) NH₂^- (b) HS^- (c) H₂O (d) CH₄ (e) NH₄⁺

Answer: (e)

11-86. Explain the following observation: Water becomes acidic when CO₂ is bubbled into it.

Answer: Because of the formation of carbonic acid, H₂CO₃

11-87. If the strength of the following acids increases from left to right

CH₃NH₂ < CH₃NH₃⁺ < C₆H₅NH₃ ⁺ < C₆H₅CO₂H < HCl

the order of increasing base strength, reading from left to right, must be:

(a) CH₃NH^- < CH₃NH₂ < C₆H₅NH₂ < C₆H₅CO₂^- < Cl^-

(b) Cl^- < C₆H₅CO₂^- < C₆H₅NH₂ < CH₃NH₂ < CH₃NH^-

(d) CH₃NH^- < C₆H₅CO₂^- < Cl^- < CH₃NH₂ < C₆H₅NH₂

(e) none of the above

Answer: (b)

11-88. What is the pH of a solution that is 1.7 x 10^-4 M in H⁺?

(a) 3.77 (b) 4.77 (c) 4.23 (d) 10.23 (e) none of these

Answer: (a)

11-89. What is the hydroxide ion concentration in a pH = 5.14 solution?

(a) 1.0 x 10^-14 (b) 1.4 x 10^-9 (c) 1.0 x 10^-7 (d) 7.2 x 10^-6 (e) 1.4 x 10^-2

Answer: (b)

[Note: 30% of the population chose answer d, for which there was a strong negative correlation, r = -0.43, while 60% chose answer b, with a strong positive correlation, r = 0.47.]

11-90. Which of the following solutions has the largest pH?

(a) 0.3 M Na₂CO₃ (b) 1 M HOAc (c) 0.3 M NH₄Cl

(d) water (e) 10^-3 M HCl

Answer: (a)

11-91. The addition of sodium formate (HCO₂Na) to a solution containing formic acid (HCO₂H: K_a = 1.8 x 10^-4) will cause:

(a) the pH to increase (b) the pH to decrease (c) no change in pH

(d) a change in the pH, the direction of which cannot be predicted.

Answer: (a)

Strong Acids

11-92. Which equation best describes an 0.10 M solution of a strong acid, such as hydroiodic acid (HI: K = 3 x 10⁹)?

(a) [H₃O⁺]_T ≈ C_HI (b) [H₃O⁺]_T ≈ [OH^-]_W (c) [H₃O⁺]_T ≈ [HI]

(d) [H₃O⁺]_T ≈ [H₂O] (e) [H₃O⁺]_T ≈ K_aC_a

Answer: (a)

Weak Acids: General

11-93. A solution is known to have a hydronium ion concentration of 1.0 x 10^-6 M. What percentage of the total H₃O⁺ ion concentration comes from the dissociation of water?

(a) 1% (b) 3% (c) 5% (d) 10% (e) 100%

[Note: less than 30% of the sample population got the correct answer: (a) ]

Answer: (a)

11-94. Assume a solution is prepared by adding 1.0 x 10^-3 moles of a weak monoprotic acid (K_a = 2.0 x 10^-4) to enough water to give a liter of solution. If you want to compute the H₃O⁺ ion concentration with an error of 5% or less, you can legitimately ignore:

(a) both the contribution of the dissociation of water and the additive ΔC term.

(b) neither the dissociation of water nor the additive ΔC term.

(d) the additive ΔC term but not the dissociation of water.

(e) you can't compute the H₃O⁺ concentration with an error of less than 5%.

Answer: (c)

11-95. When 1.0 x 10^-5 mole of HOCl (K_a = 3.5 x 10^-8) is dissolved in pure water and diluted to 1.00 L, which assumption can't be applied in the calculation of the pH of this solution? (a) that the initial concentration of HOCl is much larger than the total H₃O⁺ ion concentration

(b) that the H₃O⁺ ion concentration is much larger than K_w/[H₃O⁺]

(d) all of these assumptions are valid.

(e) none of these assumptions are valid.

Answer: (b)

11-96. When can we ignore the contribution to the total H₃O⁺ ion concentration from the dissociation of water?

(a) when K_aC_a < 1.0 x 10^-13 (b) when K_aC_a = 1.0 x 10^-13

(e) when the cows come home.

Answer: (c)

11-97. An 0.0024 M solution of boric acid (H₃BO₃: K_a = 5.8 x 10^-10) is an example of which kind of problem?

(a) a strong acid.

(b) a weak acid where we can ignore the dissociation of water and assume that ΔC is small compared with C_a.

(d) a weak acid where we can't ignore the dissociation of water but we can assume that ΔC is small compared with Ca.

(e) a weak acid where we can't ignore the dissociation of water and we can't assume that ΔC is small compared with C_a.

Answer: (b)

11-98. For a particular weak acid, HA, in pure water, as the initial concentration of the weak acid increases, the acid-dissociation equilibrium constant of that acid should:

(a) increase (b) decrease (c) remain the same

(d) increase until the ionization of water can be neglected.

(e) decrease until the ionization of water can be neglected.

Answer: (c)

11-99. What would happen if more formic acid (HCO₂H) was added to the following solution at equilibrium?

HCO₂H(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HCO₂^-(aq)

(a) [H₂O] should increase.

(b) [H₃O⁺] and [HCO₂^-] should both increase.

(d) [H₃O⁺] should increase but [HCO₂^-] should decrease.

(e) [H₃O⁺] should decrease but [HCO₂^-] should increase.

Answer: (b)

11-100. Which of the following equations is valid for a 0.10 M solution of formic acid, HCO₂H?

HCO₂H(aq) + H₂O(l) ⇌ HCO₂^-(aq) + H₃O⁺(aq) K_a = 1.8 x 10^-4

(a) [H₃O⁺]_T ≈ [OH^-]_W (b) [H₃O⁺]_W ≈ [HCO₂^-] (c) [H₃O⁺]_T ≈ [HCO₂^-]

(d) [H₃O⁺]_T = [H₂O] (e) [OH^-] ≈ [H₂O]

Answer: (c)

11-101. The value of the equilibrium constant, K_a for the dissociation of formic acid in water,

HCO₂H(aq) + H₂O(l) ⇌ HCO₂^-(aq) + H₃O⁺(aq)

would depend on:

(a) the temperature (b) the pressure (c) the pH

(d) the concentration of HOAc

(e) the concentration of the OAc^- ion.

Answer: (a)

11-102. Which of the following isn’t for the H₃O⁺ ion concentration in an 0.0100 M solution of acetic acid (K_a = 1.8 x 10^-5)?

(a) it is less than the H₃O⁺ concentration in an 0.0100 M solution of HCl.

(b) it is equal to the OAc^- ion concentration.

(d) the pH of the solution is less than 7.0.

(e) none of the above are true.

Answer: (c)

11-103. Which of the following isn’t for solutions of acetic acid (K_a = 1.8 x 10^-5)?

(a) the percent ionization increases as solutions are made more dilute.

(b) the [H₃O⁺] increases as solutions are made more dilute.

(d) its percent ionization is smaller in a 0.100 M solution containing sodium acetate than in pure water.

(e) the ionization constant is independent of concentration.

Answer: (b)

Weak Acids: Calculations

11-104. What is the percent ionization in an 0.01 M HCN solution? (HCN: K_a = 4 x 10^-10) (a) 0.002 % (b) 0.02% (c) 0.2% (d) 2% (e) 20%

Answer: (b)

11-105. Calculate the pH of an 0.10 M lactic acid solution. (K_a = 8.4 x 10^-4)

(a) ≈2 (b) ≈3 (c) ≈4 (d) ≈7 (e) ≈12

Answer: (a)

11-106. Calculate the pH of a 0.100 M solution of formic acid (HCO₂H) in water. (HCO₂H: K_a = 1.8 x 10^-4)

(a) 2.4 (b) 3.7 (c) 4.7 (d) 7 (e) 11.6

Answer: (a)

11-107. The very first disinfectant used by Joseph Lister was called “carbolic acid”. This substance is now known as phenol (PhOH). What is the H₃O⁺ ion concentration in an 0.10 M solution of phenol? (PhOH: K_a = 1.0 x 10^-10)

(a) 5.5 (b) 8.5 (c) 10 (d) 11 (e) none of the above

Answer: (a)

11-108. Hydrogen peroxide has been used as a bleach to change hair color, as a disinfectant to treat wounds, and as a rocket fuel. It is also a weak acid. Calculate the pH of an 0.018 M H₂O₂ solution. (H₂O₂: K_a = 2.2 x 10^-12)

(a) 0.6 (b) 6.7 (c) 7.3 (d) 11.7 (e) 13.4

Answer: (b)

11-109. A 0.10 M solution of a weak acid, HA, is found to be 1.50% ionized. Calculate K_a for this acid.

(a) 1.5 (b) 2.3 x 10^-1 (c) 2.3 x 10^-5 (d) 2.3 x 10^-6

(e) none of the above

Answer: (c)

11-110. A solution is prepared by dissolving 1.0 x 10^-4 moles of HOBr in water and diluting to 1.00 L. What is the pH of this solution? (HOBr: K_a = 2 x 10^-9)

(a) 1.3 (b) 5.3 (c) 6.3 (d) 8.7 (e) 12.7

Answer: (c)

11-111. A solution is prepared by dissolving 1.0 x 10^-4 moles of HOBr in water and diluting to 1.00 L. What is the pH of this solution? (HOBr: K_a = 2 x 10^-9)

(a) 5.3 (b) 6.3 (c) 7 (d) 7.7 (e) 8.7

Answer: (b)

Bases: General

11-112. What is the value of K_b for the formate ion if K_a for formic acid is 1.8 x 10^-4?

(a) K_b = K_w x K_a (b) K_b = K_a/K_w (c) K_b = K_w/K_a

(d) K_b = K_w + K_a (e) K_b = K_w - K_a

Answer: (c)-

11-113. The base-ionization constants for three bases, A(OH), B(OH), and C(OH) are 1.8 x 10^-5, 4.4 x 10^-4, and 7.4 x 10^-4, respectively. Solutions of the bases having equal molarity have pH values that decrease in the following order.

(a) A > B> C (b) B > C > A (c) C > B > A

(d) A > C > B (e) B > A > C

Answer: (c)

11-114. Which is the correct relation between K_a for an acid and K_b for its conjugate base?

(a) K_b = K_w x K_a (b) K_b = K_w/K_a (c) K_b = - log K_a

(d) K_b = 1/K_a (e) K_b = K_a/K_w

Answer: (b)

Bases: Calculations

11-115. Ammonia and the ammonium ion form a conjugate acid-base pair.

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH^-(aq)

Calculate the pH of 0.10 M NH₃ if K_a for the NH₄⁺ ion is 5.6 x 10^-10

(a) 4.7 (b) 5.1 (c) 5.7 (d) 8.9 (e) 11.1

Answer: (e)

11-116. There are many ways of “fluoridating” water. One approach involves adding a salt of the fluoride ion, such as NaF. Calculate the pH of an 0.15 M NaF solution. (HF: K_a = 7.2 x 10^-4)

(a) 2 (b) 5.8 (c) 8.2 (d) 10.9 (e) 12

Answer: (c)

11-117. Calculate the pH of a solution prepared by dissolving 1.00 x 10^-2moles of sodium hypochlorite (NaOCl) in enough water to produce a liter of solution. Hypochlorous acid is a weak monoprotic acid with K_a = 3.2 x 10^-8.

(a) 4.3 (b) 4.7 (c) 7.5 (d) 9.3 (e) 9.7

Answer: (e)

11-118. What is the OH^- ion concentration in a 0.200 M NaOAc solution? (HOAc: K_a = 1.8 x 10^-5)

(a) 1.1 x 10^-10 (b) 5.7 x 10^-10 (c) 1.8 x 10^-6

(d) 1.1 x 10^-5 (e) 1.3 x 10^-3

Answer: (d)

11-119. What is the H₃O⁺ ion concentration in a 0.1 M NH₃ solution? (K_b = 1.8 x 10^-5)

(a) 7.5 x 10^-12 (b) 3.0 x 10^-10 (c) 1.8 x 10^-6 (d) 1.3 x 10^-3

(e) none of these

Answer: (a)

11-120. HA is a weak monoprotic acid with K_a = 2.4 x 10^-6. What is [OH^-] in a 0.300 M solution of the salt, NaA?

(a) 5.8 x 10^-12 (b) 2.4 x 10^-10 (c) 3.5 x 10^-5

(d) 6.5 x 10^-5 (e) 1.6 x 10^-3

Answer: (c)

Buffers: General

11-121. Which of the following would make the best buffer?

(a) a mixture of HCl and NaCl

(b) a mixture of NaOAc and NH₃

(d) a mixture of NaOAc and NH₄Cl

(e) a mixture of NH₃ and NH₄Cl

Answer: (e)

11-122. Which of the following solutions would be an acidic buffer?

(a) 0.10 M HCl and 0.10 M NaOH

(b) 0.10 M HCl and 0.10 M NaCl

(d) 0.10 M NH₃ and 0.10 M NH₄Cl

(e) none of the above would be buffers.

Answer: (c)

11-123. When NH₄Cl is added to an 0.10 M NH₃ solution, the pH of this solution:

(a) increases (b) decreases (c) remains the same

(d) increases at first and then decreases.

(e) decreases at first and then increases.

Answer: (b)

11-124. When would the pH of a solution prepared by adding sodium formate to formic acid be equal to the pK_a of formic acid, HCO₂H?

(a) when [HCO₂H] < [HCO₂^-]

(b) when [HCO₂H] = [HCO₂^-]

(d) the pH of this buffer will never equal the pK_a of formic acid.

Answer: (b)

11-125. How would you increase the buffering capacity of a buffer made by adding NaHCO₂ to an aqueous solution of HCO₂H?

(a) increase the concentration of HCO₂H.

(b) increase the concentration of NaHCO₂.

(d) increase the ratio of the concentration of HCO₂H to NaHCO₂.

(e) increase the ratio of the concentration of NaHCO₂ to HCO₂H.

Answer: (c)

11-126. Which of the following solutions would be the best buffer when a small amount of NaOH is added to the solution?

(a) a 1.0 L solution prepared by adding 0.10 mol of carbonic acid and 0.10 mol of sodium bicarbonate.

(b) a 1.0 L solution prepared by adding 0.20 mol of carbonic acid and 0.20 mol of sodium bicarbonate.

(d) a 1.0 L solution prepared by adding 0.20 mol of hydrochloric acid and 0.20 mol of sodium hydroxide.

(e) all of these solutions would have the same buffer capacity.

Answer: (b)

11-127. Which of the following would be the best choice for preparing a pH = 8 buffer solution?

(a) A mixture of formic acid (K_a = 1.8 x 10^-4) and the formate ion.

(b) A mixture of acetic acid (K_a = 1.8 x 10^-5) and the acetate ion.

(d) A mixture of boric acid (K_a = 5.8 x 10^-10) and its conjugate base.

(e) all of these mixtures would be equally good choices for making a pH = 8 buffer.

Answer: (c)

11-128. Which of the following solutions would have the smallest pH?

(HCO₂H: K_a = 1.8 x 10^-4; NH₄⁺: K_a = 5.6 x 10^-10)

(a) 0.10 M HCO₂H (b) 0.10 M NaHCO₂ (c) 0.10 M NH₄⁺

(d) a solution that is 0.10 M in both HCO₂H and NaHCO₂

(e) a solution that is 0.10 M in both NH₄⁺ and NH₃.

Answer: (a)

11-129. Which of the following solutions will show the largest pH change when 10 mL of 1 M NaOH is added to it?

(a) 10 mL of 1 M NaOH (b) 100 mL of water

(e) 100 mL of 0.1 M HOAc and 0.1 M NaOAc

Answer: (d)

[Note, 15% of the sample fell for answer: (b), instead of answer: (d)]

Buffers: Calculations

11-130. Calculate the initial concentration of HCO₂Na you would have to add to 0.100 M HCO₂H to make a buffer with a pH of 4.00. (HCO₂H: K_a = 1.8 x 10^-4)

Answer: 0.18 M

11-131. What is the pH of a solution that is simultaneously 0.10 M in both NH₃ and the NH₄⁺ ion? (NH₄⁺: K_a = 5.6 x 10^-10)

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH^-(aq)

(a) 4.75 (b) 7.00 (c) 9.25

(d) the pH of this solution would be impossible to predict.

(e) none of the above.

Answer: (c)

11-132. What would be the effect of doubling the NH₃ concentration in the solution described in the previous question?

(a) the pH would increase.

(b) the buffering capacity would increase.

(d) all of the above statements are true.

(e) none of these statements are true

Answer: (a)

11-133. How much NaHCO₂ would you have to add to 0.10 M HCO₂H to get a buffer solution with a pH of 3.4? (HCO₂H: K_a = 2 x 10^-4)

(a) 0.010 M NaHCO₂ (b) 0.050 M NaHCO₂ (c) 0.10 M NaHCO₂

(d) 0.20 M NaHCO₂ (e) none of the above

Answer: (b)

11-134. How much NaOAc would have to be added to 1.00 L of 0.10 M HOAc to make a pH 3.00 buffer? (HOAc: K_a = 1.8 x 10^-5)

Answer: 0.0018 M

11-135. How many moles of sodium acetate (NaOAc) must be added to 500 mL of 0.25 M acetic acid (HOAc) to give a pH 4.9 buffer solution? (HOAc: K_a = 1.8 x 10^-5; assume no volume change.)

Answer: 0.18 mol

11-136. What is the [HOAc]/[OAc^-] ratio in an acetic acid/sodium acetate buffer at pH = 4.9? (HOAc: K_a = 1.8 x 10^-5)

(a) 0.31 (b) 0.70 (c) 1.4 (d) 2.4 (e) 4.90

Answer: (b)

11-137. Calculate the pH of a buffer prepared by mixing 0.10 mol of sodium formate and 0.05 mol of formic acid in 1.0 L of solution. (HCO₂H: K_a = 1.8 x 10^-4)

(a) 1.8 x 10^-4 (b) 3.44 (c) 4.05 (d) 5.31 (e) none of the above

Answer: (c)

11-138. What is the pH of a solution that is 0.01 M in CH₃NH₂ and 0.1 M in CH₃NH₃⁺?

CH₃NH₂(aq) + H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH^-(aq) K_b = 4.4 x 10^-4

(a) 2.36 (b) 3.36 (c) 9.64 (d) 10.64 (e) 11.64

Answer: (c)

11-139. If equimolar amounts of NH₃ and NH₄Cl are dissolved in 1.00 L of water, what is the H₃O⁺ ion concentration in this solution? (NH₃: K_b = 1.8 x 10^-5)

(a) 1.0 x 10^-14 (b) 5.5 x 10^-10 (c) 1.0 x 10^-7 (d) 1.8 x 10^-5

(e) it cannot be calculated from the information given.

Answer: (b)

Titration Curves

11-140. At what point in the following titration curve would the pH of the solution be equal to the pK_a of the acid?

(a) 0 mL (b) 50 mL (c) 95 mL (d) 100 mL (e) 120 mL

Answer: (b)

Polyprotic Acids

11-141. Succinic acid (HO₂CCH₂CH₂CO₂H) is an intermediate in the the Kreb's cycle used to “burn” carbohydrates, proteins and lipids. The formula for succinic acid can be abbreviated as H₂Sc. (H₂Sc: K_a1 = 6.2 x 10^-5; K_a2 = 2.3 x 10^-6)

H₂Sc(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HSc^-(aq) K_a1 = 6.9 x 10^-5

HSc^-(aq) + H₂O(l) ⇌ H₃O⁺(aq) + Sc^2-(aq) K_a2 = 2.5 x 10^-6

Which of the following assumptions may not be valid for solutions of succinic acid because the difference between K_a1 and K_a2 for H₂Sc is very small.

(a) [H₃O⁺] ≈ K_w/[OH^-] (b) [H₂Sc] < [Sc^2-] (c) [HSc^- ] ≈ [H₃O⁺]

(d) [H₂Sc] ≈ C_H2Sc (e) None of these assumptions are legitimate for this solution.

Answer: (c)

11-142. What is the pH of an 0.10 M succinic acid solution if we assume that H₂Sc is a weak acid that undergoes stepwise dissociation?

H₂Sc(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HSc^-(aq) K_a1 = 6.9 x 10^-5

HSc^-(aq) + H₂O(l) ⇌ H₃O⁺(aq) + Sc^2-(aq) K_a2 = 2.5 x 10^-6

(a) 2.1 (b) 2.6 (c) 3.3 (d) 4.2 (e) 5.6

Answer: (b)

11-143. What fraction of the HSc^- formed in the first step of the dissociation of succinic acid goes on to dissociate to form Sc^2- ions in the second step, and what does this lead us to conclude?

(a) none of the HSc^- ions dissociate further, and the stepwise dissociation assumption is legitimate.

(b) less than 1% of the HSc^- ion dissociate to form Sc^2- ions, and the stepwise dissociation assumption is legitimate.

(d) between 5 and 20% of the HSc^- ions dissociate, and the stepwise dissociation assumption is legitimate.

(e) between 5 and 20% of the HSc^- ions dissociate, and the stepwise dissociation assumption is not legitimate.

Answer: (b)

11-144. Which of the following equations accurately describes an 0.10 M H₂Gly solution?

(a) [H₃O⁺] ≈ [H₂Gly] (b) [H₃O⁺] > [H₂Gly]

(e) [H₃O⁺] > [HGly^-]

Answer: (d)

11-145. For a weak diprotic acid such as glycine, it can be shown that:

(a) [Gly^2-] ≈ C_H2Gly (b) [Gly^2-] ≈ K_a2 (c) [Gly^2-] ≈ [HGly^-]

(d) [Gly^2-] > [HGly^-] (e) [Gly^2-] ≈ [H₃O⁺]

Answer: (b)

11-146. Sulfurous acid (H₂SO₃) is an acid and sodium sulfite (Na₂SO₃) is a base. Which equation correctly describes the relationship between the equilibrium constants for these compounds?

(a) K_a1 x K_b2 = K_w (b) K_a1 x K_a2 = K_w (c) K_a2 x K_b2 = K_w

(d) K_a1 x K_a2 = 1 (e) K_a1 x K_b2 = 1

Answer: (a)

11-147. Use the relationship between K_a1 and K_a2 for H₂SO₃ and K_b1 and K_b2 for the SO₃^2- ion to predict whether an 0.10 M solution of the HSO₃^- (bisulfite) ion would be acidic, basic or neutral. (H₂SO₃: K_a1 = 1.7 x 10^-2, K_a2 = 6.4 x 10^-8)

(a) the solution would be strongly acidic, pH < 2.

(b) the solution would be moderately acidic, pH≈ 4.

(d) the solution would be slightly basic, pH ≈ 8.

(e) the solution would be strongly basic, pH > 12.

Answer: (b)

11-148. What is the pH of an 0.10 M H₂CO₃ solution? (H₂CO₃: K_a1 = 4.2 x 10^-7, K_a2 = 4.8 x 10^-11)

(a) 3.1 (b) 3.7 (c) 6.3 (d) 6.8 (e) none of the above

Answer: (b)

11-149. A diprotic acid, H₂A, has the following dissociation constants: K_a1 = 2.1 x 10^-7 and K_a2 = 4.3 x 10^-13. The H₃O⁺ ion concentration when this acid is dissolved in water is:

(a) roughly equal to the H₂A concentration.

(b) much larger than the H₂A concentration.

(d) approximately equal to the HA^- concentration.

(e) much less than the HA^- concentration.

Answer: (d)

11-150. For a weak diprotic acid, H₂A, for which K_a1 = 2.1 x 10^-7 and K_a2 = 4.3 x 10^-13 the A^2- ion concentration at equilibrium will be:

(a) approximately equal to the initial concentration of H₂A.

(b) roughly equal to K_a2.

(d) much larger than the HA^- concentration.

(e) approximately equal to the H₃O⁺ concentration

Answer: (b)

Polyprotic Bases

11-151. Which of the following equations can be used to calculate K_b1 and K_b2 for Na₂Gly from K_a1 and K_a2 for H₂Gly?

(a) K_b1 = K_{w x} K_a1 and K_b2 = K_{w x} K_a2

(b) K_b1 = K_w/K_a1 and K_b2 = K_w/K_a2

(d) K_b1 = K_w x K_a2 and K_b2 = K_w x K_a1

(e) K_b1 = K_w/K_a2 and K_b2 = K_w/K_a1

Answer: (e)

11-152. For H₂CO₃, K_a2 = 4.7 x 10^-11. Which of the following numbers is closest to the pH of a 0.200 M Na₂CO₃ solution? Hint: Only the first step in the hydrolysis of the CO₃^2- ion need be considered in this calculation.

(a) 5.9 x 10^-3 (b) 2.2 (c) 2.48 (d) 11.6 (e) 11.8

Answer: (e)

Polyprotic Acids and Bases

Use the following information to answer questions 153-155.

H₂Se: K_a1 = 1.7 x 10^-4, K_a2 = 1.0 x 10^-10

11-153. In a 0.10 M solution of H₂Se, the Se^2- concentration is:

(a) [Se^2-] = 0.10 x K_a2 = 1.0 x 10^-11 (b) [Se^2-] = K_a2 = 1.0 x 10^-10

(e) none of the above

Answer: (b)

11-154. If 0.20 moles per liter of sodium selenide is dissolved in water, the base-ionization constant for the Se^2- ion would be:

(a) K_b1 = K_a1 = 1.7 x 10^-4 (b) K_b1 = K_a2 = 1.0 x 10^-10

(e) none of the above

Answer: (d)

11-155. If sodium selenide is added to pure water, we would expect the pH to:

(a) increase (b) decrease (c) remain the same

(d) change in a direction that can't be predicted from this information.

Answer: (a)

Polyprotic Acids or Bases

11-156. KHS can act as a weak acid or a weak base.

HS^-(aq) + H₂O(l) ⇌ S^2-(aq) + H₃O⁺(aq) K_a2 = 1.3 x 10^-13

HS^-(aq) + H₂O(l) ⇌ H₂S(aq) + OH^-(aq) K_b2 = 1.0 x 10^-7

Which of the following statements is correct?

(a) a solution of 0.1 M K₂S and 0.1 M KHS will be acidic.

(b) a solution of 0.01 M H₂S will have a pH = 2.

(d) H₂S is over 90% ionized in water solution.

(e) the S^2- ion concentration in a 0.1 M KHS solution does not depend on pH.

Answer: (c)

11-157. Hydrogen sulfide (H₂S) is an acid, and sodium sulfide (Na₂S) is a base.

(H₂S: K_a1 = 1.0 x 10^-7, K_a2 = 1.3 x 10^-13)

Which of the following statements concerning NaSH is correct?

(a) NaSH is an acid because K_a1 for H₂S is much larger than K_a2.

(b) NaSH is an acid because K_a1 for H₂S is smaller than K_b1 for Na₂S.

(d) NaSH is a base because K_b2 for Na₂S is larger than K_a2 for H₂S.

(e) it is impossible to tell whether NaSH is an acid or a base from these data.

Answer: (d)

11-158. H₂Gly is an acid and Na₂Gly is a base. On the basis of the values of K_a1 and K_a2 for H₂Gly and the values of K_b1 and K_b2 for Na₂Gly, would an 0.10 M NaHGly solution be acidic, basic, or neutral? (H₂Gly: K_a1 = 4.5 x 10^-3, K_a2 = 2.5 x 10^-10)

(a) slightly acidic (b) slightly basic (c) neutral (pH = 7)

(d) there is no way to predict whether NaHGly is an acid or base.

Answer: (a)