Chapter 15: Acid-Base Equilibria 397

Chapter 15: Acid-Base Equilibria 397

Chapter 15

Acid-Base Equilibria

1. Calculate the [H+] in a solution that is 0.17 M in NaF and 0.25 M in HF. (Ka = 7.2 ´ 10–4)

a) 7.2 ´ 10–4 M

b) 1.5 M

c) 1.1 ´ 10–3 M

d) 0.20 M

e) 4.9 ´ 10–4 M

Ans: c Algorithm: Yes Chapter/Section: 15.1 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: common-ion effect

2. For a solution equimolar in HCN and NaCN, which statement is false?

a) This is an example of the common ion effect.

b) The [H+] is larger than it would be if only the HCN was in solution.

c) The [H+] is equal to the Ka.

d) Addition of more NaCN will shift the acid dissociation equilibrium of HCN to the left.

e) Addition of NaOH will increase [CN–] and decrease [HCN].

Ans: b Algorithm: No Chapter/Section: 15.1 Difficulty: Easy Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: common-ion effect

3. What will happen if a small amount of hydrochloric acid is added to a 0.1 M solution of HF?

a) The percent ionization of HF will increase.

b) The percent ionization of HF will decrease.

c) The percent ionization of HF will remain unchanged.

d) Ka for HF will increase.

e) Ka for HF will decrease.

4. What will happen if a small amount of sodium hydroxide is added to a 0.1 M solution of ammonia?

a) Kb for ammonia will increase.

b) Kb for ammonia will decrease.

c) The percent ionization of ammonia will increase.

d) The percent ionization of ammonia will decrease.

e) The percent ionization of ammonia will remain unchanged.

Ans: d Algorithm: No Chapter/Section: 15.1 Difficulty: Easy Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: common-ion effect

5. 15.0 mL of 0.50 M HCl is added to a 100.-mL sample of 0.462 M HNO2 (Ka for HNO2 = 4.0 ´ 10–4). What is the equilibrium concentration of NO2– ions?

a) 2.5 ´ 10–3 M

b) 1.6 ´ 10–4 M

c) 4.0 ´ 10–1 M

d) 5.0 ´ 10–2 M

e) none of these

Ans: a Algorithm: Yes Chapter/Section: 15.1 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: common-ion effect

6. 15.0 mL of 0.50 M NaOH is added to a 100.-mL sample of 0.442 M NH3 (Kb for NH3 = 1.8 ´ 10–5). What is the equilibrium concentration of NH4+ ions?

a) 1.0 ´ 10–2 M

b) 6.9 ´ 10–6 M

c) 3.8 ´ 10–1 M

d) 1.1 ´ 10–4 M

e) none of these

Ans: d Algorithm: Yes Chapter/Section: 15.1 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: common-ion effect

7. What is the percent dissociation of HNO2 when 0.074 g of sodium nitrite is added to 115.0 mL of a 0.068 M HNO2 solution? Ka for HNO2 is 4.0 × 10–4.

a) 16%

b) 0.29%

c) 4.3%

d) 0.062%

e) 7.7%

40. In titrating 0.20 M hydrochloric acid, HCl, with 0.20 M NaOH at 25°C, the solution at the equivalence point is

a) 0.20 M NaCl

b) very acidic

c) slightly acidic

d) 0.10 M HCl and 0.20 M NaOH

e) 0.10 M NaCl

Ans: e Algorithm: Yes Chapter/Section: 15.4 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

41. One milliliter (1.00 mL) of acid taken from a lead storage battery is pipetted into a flask. Water and phenolphthalein indicator are added, and the solution is titrated with 0.55 M NaOH until a pink color appears; 12.0 mL are required. The number of grams of H2SO4 (formula weight = 98) present in one liter of the battery acid is:

a) 647

b) 323

c) 30

d) 1294

e) 54

Ans: b Algorithm: Yes Chapter/Section: 15.4 Difficulty: Easy Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

42. You are given 5.00 mL of an H2SO4 solution of unknown concentration. You divide the 5.00-mL sample into five 1.00-mL samples and titrate each separately with 0.1000 M NaOH. In each titration the H2SO4 is completely neutralized. The average volume of NaOH solution used to reach the endpoint is 12.6 mL. What was the concentration of H2SO4 in the 5.00-mL sample?

a) 1.26 M

b) 3.15 M

c) 0.630 M

d) 0.126 M

e) 6.30 M

Ans: c Algorithm: Yes Chapter/Section: 15.4 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

43. What is the molarity of a sodium hydroxide solution if 27.9 mL of this solution reacts exactly with 22.30 mL of 0.253 M sulfuric acid?

a) 0.202 M

b) 0.809 M

c) 7.06 M

d) 0.404 M

e) 0.225 M

Ans: d Algorithm: Yes Chapter/Section: 15.4 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

44. If 25.0 mL of 0.451 M NaOH solution is titrated with 0.253 M H2SO4, the flask at the endpoint will contain (besides the indicator phenolphthalein) as the principal components:

a) sodium hydroxide, sulfuric acid, and water

b) dissolved sodium sulfate and water

c) sodium hydroxide, sodium sulfate, and water

d) dissolved sodium sulfate, sulfuric acid, and water

e) precipitated sodium sulfate and water

Ans: b Algorithm: No Chapter/Section: 15.4 Difficulty: Easy Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

45. A 21.0-mL sample of tartaric acid is titrated to a phenolphthalein endpoint with 20. mL of 1.0 M NaOH. Assuming tartaric acid is diprotic, what is the molarity of the acid?

a) 1.0 M

b) 0.48 M

c) 2.0 M

d) 0.95 M

e) impossible to determine

Ans: b Algorithm: Yes Chapter/Section: 15.4 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of polyprotic acids

46. If 25 mL of 0.750 M HCl are added to 100. mL of 0.346 M NaOH, what is the final pH?

a) 13.10

b) 0.90

c) 13.44

d) 0.56

e) 7.00

Ans: a Algorithm: Yes Chapter/Section: 15.4 Difficulty: Moderate Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

47. A 50.00-mL sample of 0.100 M KOH is titrated with 0.287 M HNO3. Calculate the pH of the solution after 52.00 mL of HNO3 is added.

a) 12.99

b) 0.83

c) 1.01

d) 13.17

e) none of these

48. A solution of hydrochloric acid of unknown concentration was titrated with 0.37 M NaOH. If a 100.-mL sample of the HCl solution required exactly 10. mL of the NaOH solution to reach the equivalence point, what was the pH of the HCl solution?

a) 12.6

b) 1.4

c) –0.6

d) 2.9

e) 5.7

49. A titration of 200.0 mL of 1.84 M H2A was done with 1.00 M NaOH. For the diprotic acid H2A, Ka1 = 2.5 ´ 10–5, Ka2 = 3.1 ´ 10–9. Calculate the pH before any 1.00 M NaOH has been added.

a) 11.83

b) 4.34

c) 9.66

d) 8.67

e) 2.17

Ans: e Algorithm: Yes Chapter/Section: 15.4 Difficulty: Easy Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of polyprotic acids

50. A titration of 200.0 mL of 1.00 M H2A was done with 1.28 M NaOH. For the diprotic acid H2A, Ka1 = 2.5 ´ 10–5, Ka2 = 3.1 ´ 10–9. Calculate the pH after 100.0 mL of 1.28 M NaOH have been added.

a) 9.15

b) 8.76

c) 5.24

d) 9.70

e) 4.85

51. A titration of 200.0 mL of 1.00 M H2A was done with 1.05 M NaOH. For the diprotic acid H2A, Ka1 = 2.5 ´ 10–5, Ka2 = 3.1 ´ 10–9. Calculate the pH after 600.0 mL of 1.05 M NaOH have been added.

a) 13.459

b) 0.541

c) 13.730

d) 0.270

e) 9.459

52. Consider the titration of 300.0 mL of 0.498 M NH3 (Kb = 1.8 ´ 10–5) with 0.500 M HNO3. After 150.0 mL of 0.500 M HNO3 have been added, the pH of the solution is:

a) 4.75

b) 11.25

c) 6.25

d) 9.25

e) none of these

53. Consider the titration of 300.0 mL of 0.700 M NH3 (Kb = 1.8 ´ 10–5) with 0.700 M HNO3. How many milliliters of 0.700 M HNO3 are required to reach the stoichiometric point of the reaction?

a) 3.50 ´ 102 mL

b) 4.00 ´ 102 mL

c) 7.00 ´ 102 mL

d) 3.00 ´ 102 mL

e) none of these

54. Consider the titration of 300.0 mL of 0.250 M NH3 (Kb = 1.8 ´ 10–5) with 0.250 M HNO3. At the stoichiometric point of this titration, the pH is:

a) 4.93

b) 2.82

c) 5.08

d) 4.74

e) 7.00

55. Consider the titration of 500.0 mL of 0.200 M NaOH with 0.800 M HCl. How many milliliters of 0.800 M HCl must be added to reach a pH of 13.000?

a) 55.6 mL

b) 24.6 mL

c) 18.5 mL

d) 12.9 mL

e) 4.32 mL

Ans: a Algorithm: No Chapter/Section: 15.4 Difficulty: Difficult Keyword 1: Chemistry Keyword 2: general chemistry Keyword 3: acids and bases Keyword 4: solutions of a weak acid or base with another solute Keyword 5: acid-base titration curve Keyword 6: titration of a strong acid by a strong base

56. What quantity of NaOH(s) must be added to 2.00 L of 0.538 M HCl to achieve a pH of 13.00? (Assume no volume change.)

a) 0.88 mol

b) 1.28 mol

c) 0.20 mol

d) 1.00 ´ 10–13 mol

e) none of these

57. A 50.0-mL sample of 0.10 M HNO2 (Ka = 4.0 ´ 10–4) is titrated with 0.12 M NaOH. The pH after 25.0 mL of NaOH have been added is