CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 381
11.5.10Hydrolysis of the salt of a weak acid and a weak base
The salt of a weak monoacidic base and a weak monobasic acid BA is a strong electrolyte and
it dissociates completely according to the equation
BA−→ B++ A−. (11.76)Two hydrolytic reactions proceed in the system
B++ H 2 O = BOH + H+, (11.77)
A−+ H 2 O = HA + OH−. (11.78)In addition to them, the dissociation of water, i.e. the reaction (11.47), also has to be considered.
Let the equilibrium constant of reaction (11.77) be denotedKh, 1 and the equilibrium con-
stant of reaction (11.78) be denotedKh, 2. Then we write
Kh, 1 =aH+aBOH
aB+aH 2 O, (11.79)
Kh, 2 =aOH−aHA
aA−aH 2 O. (11.80)
In dilute solutions the equation simplifies to
Kh, 1 =cH+cBOH
cB+1
cst, (11.81)
Kh, 2 =cOH−cHA
cA−1
cst. (11.82)
When calculating the equilibrium composition we solve a set of three equations: (11.81),
(11.82), and (11.49), with the latter describing the dissociation of water.