Page 62 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNOAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

Application of these theoretical considerations to situations encountered in
practice may be illustrated by numerical examples.
Example 6. Calculate the concentrations of HS- and S2- in a solution of
hydrogen sulphide.
A saturated aqueous solution of hydrogen sulphide at 25 OC, at atmospheric
pressure, is approximately O.lM, and for H2S the primary and secondary
dissociation constants may be taken as 1.0 x 10-7molL-' and 1 x 10-'4molL-'
respectively.
In the solution the following equilibria are involved:
H2S + H20 e HS- + H30+; KI = [H+][HS-]/[H2S] (d)
HS- + H20 e S2- + H30+; K2 = [H+][S2-]/[HS-] (e)
H20=H+ +OH-
Electroneutrality requires that the total cation concentration must equal total
anion concentration and hence, taking account of charge numbers,
CH+] = [HS-] + 2[S2-] + [OH-] (f)
but since in fact we are dealing with an acid solution, CH+] > > [OH -1
and we can simplify equation (e) to read
[H'] = [HS-] + 2[S2-] (9)

The 0.1 mol H2S is present partly as undissociated H2S and partly as the ions
HS- and S2-, and it follows that

The very small value of K2 indicates that the secondary dissociation and
therefore [S2-] are extremely minute, and ignoring [S2-] in equation (g)
we are left with the result

Since KI is also small, CH+] << [H2S] and so equation (h) can be reduced to

Using these results i11 equation (d) we find

From equation (e) it then follows that
(1.0 10-~)[s~-]/(i.o 10-4) = 1 10-14
and [S2-] = 1 x 10-'4molL-'.

2.15 COMMON ION EFFECT

The concentration of a particular ion in an ionic reaction can be increased by
the addition of a compound which produces that ion upon dissociation. The