Solutions for Acid-Base Equilibria & Titrations Exercises
1.
HA-(aq) + H2O(l)
º H3O+(aq) + A2-(aq)
Ka = 10-8
HA-(aq) + H2O(l)
º H2A(aq) + OH-(aq)
Kb = 10-11
Since Ka>>Kb, the acid dissociation reaction is favored. However, the H3O+
produced in this reaction will react with the OH- formed in the base hydrolysis and
drive this reaction further to the right than might be expected from the value of Kb.
2.
a)
H2M(aq) + H2O(l)
init
0.100 M
equil 0.100 - x
º H3O+(aq) + HM-(aq)
~10-7 M
0
~x
x
Y x = 0.0119 M (x << 0.1 assumption is not valid! - use successive
approximations)
x2 = 0.0112
x3 = 0.0112 M = [H3O+] = [HM-] Y pH = 1.95 and [H2M] = 0.100 M - 0.0112 M =
0.089 M
Since Ka2 << Ka1, the second acid dissociation should not change these
concentrations much.
HM-(aq) + H2O(l)
init
0.0112 M
equil ~0.0112 M
º H3O+(aq) + M2-(aq)
0.0112 M
0
~0.0112 M
x Solutions for Acid-Base Equilibria & Titrations Exercises
b) 0.100 M NaHM (amphiprotic system)
HM-(aq) + H2O(l)
º H3O+(aq) + M2-(aq)
HM-(aq) + H2O(l)
º H2M(aq) + OH-(aq)
For this amphiprotic substance HM- (assuming [HM-] = 0.100 M)
[H3O+] = 5.3 x 10-5 M Y pH = 4.28 Solutions for Acid-Base Equilibria & Titrations Exercises
c) 0.100 M Na2M
M2-(aq) + H2O(l)
HM-(aq) + H2O(l)
º HM-(aq) + OH-(aq)
º H2M(aq) + OH-(aq)
Since Kb2 << Kb1, treat salt as monobasic
M2-(aq) + H2O(l)
º HM-(aq) + OH-(aq)
0.100 M
0
~10-7 M
equil 0.100 - x
x
~x
init
Y x = 2.23 x 10-5 M = [HM-] = [OH-] Y pOH = 4.65 and pH = 9.35
also, [M2-] = 0.100 M Solutions for Acid-Base Equilibria & Titrations Exercises
3.
For acid HA, pKa = 7.00 (Ka = 1.0 x 10-7)
When pH = pKa, [A-] = [HA]
If pH < pKa, [HA] > [A-]
If pH > pKa, [A-] > [HA]
4.
a)
at pH = 6.00, [H3O+] = 1.0 x 10-6 Y [HA] > [A-]
b)
at pH = 8.00, [H3O+] = 1.0 x 10-8 Y [A-] > [HA]
c)
at pH = 6.40, [H3O+] = 3.98 x 10-7 Y [A-] / [HA] = 0.25
For the HCO3-/CO32- buffer, if pH = 10.00 Y [H3O+] = 1.0 x 10-10 M and 5.00 g
NaHCO3/100 mL Y [HCO3-] = 0.5952 M
HCO3-(aq) + H2O(l)
init
0.5952 M
equil ~0.5952 M
º H3O+(aq) + CO32-(aq)
~10-7 M
x
1.0 x 10-10 M
~x
Solutions for Acid-Base Equilibria and Titrations Exercises
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