Page 71 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

HYDROLYSIS CONSTANT AND DEGREE OF HYDROLYSIS 2.19

must CO-exist with the hydrolytic equilibrium:
A- + H20=HA + OH-
Hence the two relationships:
[H'] x [OH-] = K, and [H'] x [A-]/[HA] = Ka
must hold in the same solution as:
[OH-] x [HA]/[A-] = Kh
K, [H'] x [OH-] x [HA] [OH-] x [HA]
-
But - - = Kh
=
Ka CH'I x CA-] CA-]
therefore Kw/Ka = Kh
or pKh = pK, - pKa
The hydrolysis constant is thus related to the ionic product of water and
the ionisation constant of the acid. Since Ka varies slightly and K, varies
considerably with temperature, Kh and consequently the degree of hydrolysis
will be largely influenced by changes of temperature.
The hydrogen ion concentration of a solution of a hydrolysed Salt can be
readily calculated. The amounts of HA and of OH - ions formed as a result of
hydrolysis are equal; therefore, in a solution of the pure salt in water,
[HA] = [OH-]. If the concentration of the Salt is cm01 L-', then:
[HA] x [OH -1 - [OH -1 =K - Kw
CA - 1 -. C h-- Ka

and [OH-] = ,/c.K,/K,
or [H'] = Jm, [H'] = K,/[OH-]
since
and pH = ip~, + $pKa + +log c
To be consistent we should use pc = -log c so that the equation becomes:

Equation (14) can be employed for the calculation of the pH of a solution
of a Salt of a weak acid and a strong base. Thus the pH of a solution of sodium
benzoate (0.05 mol L-') is given by:

(Benzoic acid: Ka = 6.37 x 10-5molL-'; pKa = 4.20)
Such a calculation will provide useful information as to the indicator which
should be employed in the titration of a weak acid and a strong base (see
Section 10.13).
Example II. Calculate: (i) the hydrolysis constant, (ii) the degree of hydrolysis,
and (iii) the hydrogen ion concentration of a solution of sodium acetate
(0.01 mol L-') at the laboratory temperature.