Page 72 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS
P. 72
2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION
The degree of hydrolysis x is given by:
xz
K, =
(1 -x)V
Substituting for K, and V ( = l/c), we obtain:
Solving this quadratic equation, x = 0.0002 38 or 0.0238 percent.
If the solution were completely hydrolysed, the concentration of acetic
(ethanoic) acid produced would be 0.01 mol L-'. But the degree of hydrolysis
is 0.0238 percent, therefore the concentration of acetic acid is 2.38 x 10-6molL-'.
This is also equal to the hydroxide ion concentration produced, i.e. pOH = 5.62.
The pH may also be calculated from equation (14):
Case 2. Salt of a strong acid and a weak base. The hydrolytic equilibrium is
represented by:
By applying the Law of Mass Action along the lines of Case 1, the following
equations are obtained:
[H'] x [MOH] [Acid] x [Base] K,
-
-
K, = - --
CM'] [Unhydrolysed salt] K,
K, is the dissociation constant of the base. Furthermore, since [MOH] and
[H +] are equal:
[H'] x [MOH] [H+I2 K,
K, = - - -
CM'] C Kb
Equation (15) may be applied to the calculation of the pH of solutions of
salts of strong acids and weak bases. Thus the pH of a solution of ammonium
chloride (0.2 mol L - ' ) is:
(Ammonia in water: K, = 1.8 x 10-'mol L-'; pK, = 4.74)
Case 3. Salt of a weak acid and a weak base. The hydrolytic equilibrium is
expressed by the equation:
