Page 70 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

The hydrolysis of the sodium Salt of a weak acid can be treated similarly.
Thus for a solution of sodium acetate
CH3COOc +H20=CH3COOH +OH-
the hydrolysis constant is
[CH3COOH][OH-]/[CH3COO-] Kh = K,/Ka
=
where Ka is the dissociation constant of acetic (ethanoic) acid.

2.19 HYDROLYSIS CONSTANT AND DEGREE OF HYDROLYSIS
Case 1. Salt of a weak acid and a strong base. The equilibrium in a solution of
salt MA may be represented by:
A-+H20=OH-+HA
Applying the Law of Mass Action, we obtain:

where K, is the hydrolysis constant. The solution is assumed to be dilute so
that the activity of the un-ionised water may be taken as constant, and the
approximation that the activity coefficient of the un-ionised acid is unity
and that both ions have the same activity coefficient may be introduced.
Equation (12) then reduces to:

[OH-] x [HA]
K, =
CA - 1
This is often written in the form:
[Base] x [Acid]
K, =
[Unh ydrolysed salt ]
The free strong base and the unhydrolysed Salt are completely dissociated and
the acid is very little dissociated.
The degree of hydrolysis is the fraction of each mole of anion A - hydrolysed
at equilibrium. Let 1 mole of Salt be dissolved in V L of solution, and let x
be the degree of hydrolysis. The concentrations in mol L-' are:
[HA] = [OH-] = x/V; [A-] = (1 - x)/V
Substituting these values in equation (13):

[OH -1 x [HA] - x/ V x x/ V x2
K, = - - -
CA - 1 (1-x)/V (1-x)V
This expression enables us to calculate the degree of hydrolysis at the dilution
V; it is evident that as V increases, the degree of hydrolysis x must increase.
The two equilibria:
H20=H++OH- and HAeH++A-