Acid-Base Equilibrium Problems
1. Write equations which represent the dissociation of each of these acids or bases in aqueous
solution. Use a single arrow in the case of a strong acid or base, and a double arrow to represent
the equilibrium condition that exists in the solution of a weak acid or base. Show each step of
dissociation for polyprotic acids.
a) KOH
b) H3AsO4
c) HClO4
d) HCN(aq)
e) C6H5NH2 (a weak base)
2. Benzoic acid, HC6H5CO2, is an organic acid whose sodium salt, NaC6H5CO2, has long been used as a
safe food additive to protect beverages and many foods against harmful yeasts and bacteria. The
acid is monoprotic. Write the Ka expression for this acid.
3. Write the equilibrium equations and the Kb expressions for each of the following bases.
a) CN- (cyanide ion) b) C2H3O2- (acetate ion) c) C6H5NH2 (aniline) d) H2O
4. Find the pH and % ionization of a 0.065 M solution of formic acid, (Ka=1.8 x 10-4). [2.47, 5.2%]
5. Find the pH of a 0.325 M acetic acid solution, given the Ka is 1.8 x 10-5. [2.62, 0.74%]
6. Find the pH of a solution that contains 0.0034 M lactic acid (HC3H5O3) (Ka =1.4 x 10-4). [3.20]
7. Find the hydronium ion concentration and pH for a 0.056 M solution of propanoic acid (Ka = 1.4 x
10-5). [ H3O+=8.9x10-4 pH=3.05]
8. Find the pH of a 0.600 M solution of methylamine CH3NH2. Kb = 4.4 x 10–4. [12.20]
9. What is the pH of a 5.6x10-4 M butanoic solution, given the pKa = 4.82. [4.08]
10. Calculate [H3O+], pH and % ionization for a 0.50 mol/L HCN solution. [1.8x10-5,4.75, 3.6x10-3 %]
11. Calculate the [OH-], pH and % ionization for 0.25 mol/L ammonia solution. [2.1x10-3, 11.32, 0.84%]
12. Hydrazine, N2H4, used as a rocket fuel, is a weak base. A 0.15 M solution has a pH of 10.70. What
is the Kb and pKb for hydrazine and the pKa of its conjugate acid? [Kb=1.7x10-6, pKb =5.77, pKa Conj
=8.23]
13. If the pH of a weak base solution is 9.50 and the original concentration of base was 0.30 M what is
the pOH, the concentration of OH-, the equilibrium concentration of the base and the Kb of the
base? [pOH = 4.50, [OH-]= 3.2x10-5, [B]=0.30 M, Kb=3.4x10-9]
14. If the pH of a hypochlorous acid solution is 4.17, what is the initial concentration? [0.16 M]
15. If the pH of a solution is equal to 8.70 and the Kb=9.6x10-7 what was the original concentration of
the base? [3.1x10-5 M]
16. A nitrous acid solution is 1.34% ionized at equilibrium. What is the hydronium ion concentration,
pH and initial acid concentration? [ [H3O+]=0.0531 M, pH=1.274, [HNO2]initial=3.96 M ]
17. A solution of ethylamine is 1.25% ionized at equilibrium. What is the hydroxide ion concentration
and initial base concentration? [[OH-]=0.0340 M, [CH3CH2NH2]initial=2.72 M]
18. Calculate the pH of the following solutions,
a) 0.36 M sodium acetate solution [9.15]
c) 0.16 M calcium formate solution [8.62]
b) 0.25 M ammonium nitrate
d) 0.45 M potassium hypochlorite
[4.92]
[10.59]
19. A 27.0 mL sample of 0.28 M acetic acid is titrated with 0.15 M calcium hydroxide.
a) Calculate the volume of calcium hydroxide required to reach the equivalence point. [25mL]
b) Calculate the pH at equivalence.
[8.96]
20. A 175 mL sample of 0.30 M ammonia is titrated with 0.45 M hydrochloric acid.
a) Calculate the volume of hydrochloric acid required to reach the equivalence point. [120mL]
b) Calculate the pH at equivalence.
[9.00]
21. A 225 mL sample of 0.24 M formic acid is titrated with 0.30 M sodium hydroxide.
a) Calculate the volume of hydrochloric acid required to reach the equivalence point. [180mL]
b) Calculate the pH at equivalence.
[8.43]
22. A 180 mL sample of 0.16 M hydrazine is titrated with 0.12 M nitric acid.
a) Calculate the volume of nitric acid required to reach the equivalence point.
b) Calculate the pH at equivalence.
23. A 375 mL sample of 0.25 M hydrofluoric acid is titrated with 0.15 M potassium hydroxide.
a) the volume of potassium hydroxide needed to reach the equivalence point
[630 mL]
b) the pH at equivalence.
[8.07]
24. If 125 mL of 0.15 M hydrochloric acid is needed for 95 mL of ammonia to reach equivalence, find:
a) the concentration of the original ammonia solution.
[0.20 M]
b) the pH of the solution at the equivalence point.
[5.16]
25. If 230 mL of 0.35 M cyanic acid (Ka=3.5x10-4) is titrated with 0.25 M calcium hydroxide. Calculate:
a) the pH of the acid sample before titration.
[1.96]
b) the volume of calcium hydroxide needed to reach the equivalence point.
[160 mL]
c) the pH of the solution at the equivalence point.
[8.39]
26. A 65 mL sample of 0.45 M methylamine is titrated with 0.45 M nitric acid. Calculate:
a) the pH of the base sample before titration.
[12.15]
b) the volume of nitric acid needed to reach the equivalence point.
[65 mL]
c) the pH of the solution at the equivalence point.
[5.66]
27. If 65 mL of 0.20 M calcium hydroxide is need to titrate 85 mL of an unknown concentration of
nitrous acid to equivalence point, calculate:
a) the concentration of the original nitrous acid solution.
[0.31 M]
b) the pH of the solution at the equivalence point.
[8.19]
Buffer Questions
For each of the following questions calculate the pH of a) the initial buffer, b) after the addition of the
H3O+ to the initial buffer and c) after the addition of the OH- to the initial buffer.
Acid or Base
Conjugate
Volume (L)
moles of H3O+
moles of OH0.300 mol of NH3
0.600 mol of NH4NO3
3.0
0.150
0.150
0.400 mol HC2H3O2
0.400 mol NaC2H3O2
2.0
0.200
0.100
0.250 mol H2CO3
0.200 mol NaHCO3
1.0
0.050
0.060
0.200 mol CH3NH2
0.150 mol CH3NH3NO3
3.0
0.020
0.030
0.900 mol HCN
0.800 mol NaCN
2.0
0.200
0.300
1.50 mol benzoic acid
1.20 mol sodium benzoate
3.0
0.600
0.300
0.450 mol HClO
0.350 mol KClO
1.0
0.250
0.15
Answers
Question
1
2
3
4
5
6
7
Initial pH
8.95
4.74
6.26
10.76
9.15
4.10
7.43
After H3O+
8.56
4.27
6.06
10.65
8.96
3.66
6.70
After OH9.26
4.96
6.49
10.92
9.47
4.30
7.77