Page 64 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

completely dissociated sodium acetate are:

[Na + ] = 0.1, and [CH, CO0 -1 = 0.1 mol L- ' respectively.
The acetate ions from the Salt will tend to decrease the ionisation of the acetic
acid, and consequently the acetate ion concentration derived from it. Hence we
may write [CH,COO-] = 0.1 for the solution, and if cc' is the new degree of
ionisation, [H +] = cc'c = O.lcc1, and [CH,COOH] = (1 - ccl)c = 0.1, since cc' is
negligibly small.
Substituting in the mass action equation:

or cc' = 1.75 x

The addition of a tenth of a mole of sodium acetate to a 0.1 M solution of
acetic acid has decreased the degree of ionisation from 1.32 to 0.018 percent,
and the hydrogen ion concentration from 0.001 32 to 0.000018 mol L-'.
Example 9. What effect has the addition of 0.5 mol of ammonium chloride
to 1 L of 0.1 M aqueous ammonia solution upon the degree of dissociation of
the base?
(Dissociation constant of NH, in water = 1.8 x IO-' mol L- ')

In 0.1 M ammonia solution cc = J1.8 x 10-'/0.1 = 0.0135. Hence [OH -1 =
0.001 35, [NH;] = 0.001 35, and [NH,] = 0.0986 mol L-'. Let cc' be the degree
of ionisation in the presence of the added ammonium chloride. Then
[OH -1 = cc'c = 0.1 cc', and [NH,] = (1 - ccl)c = 0.1, since cc' may be taken as
negligibly small. The addition of the completely ionised ammonium chloride
will, of necessity, decrease the [NH;] derived from the base and increase
[NH,], and as a first approximation [NH;] = 0.5.
Substituting in the equation:
[NH;] x [OH-] - 0.5 x 0.1 cc'
- = 1.8 x IO-'
CNH31 o. 1
cc' = 3.6 x IO-' and [OH -1 = 3.6 x mol L- '

The addition of half a mole of ammonium chloride to 1 litre of a 0.1 M
solution of aqueous ammonia has decreased the degree of ionisation from 1.35
to 0.0036 percent, and the hydroxide ion concentration from 0.001 35 to
0.000 0036 mol L - '.

2.16 THE IONlC PRODUCT OF WATER

Kohlrausch and Heydweiller (1894) found that the most highly purified water
that can be obtained possesses a small but definite conductivity. Water must