Chem 22(C)
Assigned Problems in S., B., R., Chapter 11, p 530
17. Identify conjugate acid base pairs
19. Write Lewis structures for the following BrØnsted acids and
their conjugate bases.
(a) formic acid, HCO2H (b) methanol, CH3OH
22. Identify the conjugate base of each of the following BrØnsted acids.
(a) HPO42- (b) H2PO4- (c) HCO3- (d) HS-
23. Identify the conjugate acid of each of the following BrØnsted bases.
(a) O2- (b) OH- (c) H2O (d) NH2-
34. Use the table of acid dissociation equilibrium constants in Appendix B.8
to classify
the following acids as either strong or weak.
(a) acetic acid, CH3CO2H (b) boric acid, H3BO3
© chromic acid, H2CrO4 (d) formic acid, HCO2H
(e) hydrobromic acid, HBr
35. Which of the following is the weakest BrØnsted acid?
(a) H2S2O3, Ka = 0.3
(b) H2CrO4, Ka = 9.6
(c) H3BO3, Ka = 7.3 x 10-10
(d) C6H5OH, Ka = 1.0 x 10-10
47. Which of the following solutions is the most acidic?
(a) 0.10 M acetic acid, pH = 2.9
(b) 0.10 M hydrogen sulfide, pH = 4.1
(c) 0.10 M sodium acetate, pH = 8.4
(d) 0.10 M ammonia, pH = 11.1
49. Which of the following compounds could dissolve in water to give a 0.10
M solution
with a pH of about 5?
(a) NH3
(b) NaCl
(c) HCl
(d) KOH
(e) NH4Cl
51. The dissociation of water is an endothermic reaction.
2 H2O(1) H3O+(aq) + OH-(aq) DHº = 55.84 kJ/molrxn
Use Le Châtelier's principle to predict what should happen to the fraction of
water
Molecules that dissociate into ions as the temperature of water increases.
52. Use Le Châtelier's principle to explain why adding either an acid or
a base to water
suppresses the dissociation of water.
55. Approximate the pH and pOH of a 0.035 M HCl solution.
60. Which of the following solutions is the most acidic?
(a) 0.10 M CH3CO2H, Ka = 1.8 x 10-5
(b) 0.10 M HCO2H, Ka = 1.8 x 10-4
(c) 0.10 M ClCH2CO2H, Ka = 1.4 x 10-3
(d) 0.10 M Cl2CHCO2H, Ka = 5.1 x 10-2
61. Which of the following compounds is the strongest base?
(a) CH3CO2- (for CH3O2H, Ka = 1.8 x 10-5)
(b) HCO2- (for HCO2H, Ka = 1.8 x 10-4)
(c) ClCH2CO2- (for ClCH2CO2H, Ka = 1.4 x 10-3)
(d) Cl2CHCO2- (for Cl2CHCO2H, Ka = 5.1 x 10-2)
66. Nitric acid is often grouped with sulfuric acid and hydrochloric acid as one
of the
strong acids. Calculate the pH of 0.10 M nitric acid, assuming that it is a strong
acid
that dissociates completely. Calculate the pH of the solution using the value of
Ka for
the acid (for HNO3, Ka = 28).
73, Hydrogen cyanide (HCN) is a gas that dissolves in water to form hydrocyanic
acid.
Approximate the H3O+, HCN, and CN- concentrations in a 0.174 M solution of hy-
Drocyanic acid. (For HCN, Ka = 6 x 10-10.)
75. Approximate the concentration of acetic acid that would give an H3O+ ion concen-
tration of 2.0 x 10-3 M. (For HOAc, Ka = 1.8 x 10-5.)
76. Calculate the value of Ka for ascorbic acid (vitamin C) if 2.8% of the
ascorbic acid
molecules in a 0.100 M solution dissociate.
79. Use the relationship between Ka for an acid and Kb for its conjugate base
to explain
why strong acids have weak conjugate bases and weak acids have strong conjugate
bases.
80. Approximate the HCO2H, OH-, and HCO2- ion concentrations in a solution
that
contains 0.020 mol of sodium formate (NaHCO2) in 250 mL of solution. (For
HCO2H, Ka = 1.8 x 10-4.)
82. Approximate the pH of a 0.756 M solution of NaOAc. (For HOAc, Ka = 1.8 x 10-5.)
89. Explain how buffers resist changes in pH.
90. Explain why a mixture of HOAc and NaOAc is an acidic buffer, but a mixture
of
NH3 and NH4Cl is a basic buffer. (For HOAc, Ka = 1.8 x 10-5; for NH4+,
Ka = 5.6 x 10-10.)
95. Determine the approximate pH of a buffer formed by a solution containing 0.10
M
formic acid and 0.1 M sodium formate.
97. Determine the approximate pH of a buffer formed by adding 15 g of benzoic
acid
and 10 g of sodium benzoate to water to form 1.0 liter of solution.
100. Explain why pure (nonpolluted) rainwater has a pH of 5.6. Explain why
boiling
water to drive off the CO2 raises the pH. Explain why the presence of SO2, SO3,
NO, NO2, and similar compounds in the atmosphere gives rise to acid rain.
105. The pH of a 0.10 M solution of formic acid is 2.37.
Formic acid
(a) Which of the two hydrogens in the formic acid structure is the "acidic"
Hydrogen. Explain.
(b) Write a chemical equation that describes what happens when formic acid
is
placed in water.
(c) Calculate the Ka of formic acid.
(d) Estimate the pH of a 0.10 M solution of the following acid in water. Explain
your estimate.
11A.4 One form of glycine, the simplest amino acid found in proteins,
is a diprotic acid with the formula 
. Denoting this by H2G+ its
stepwise dissociations may be written as
