Question

A 0.500 M solution of a weak base has a pH of 9.00. What is the base hydrolysis constant, Kb, for the weak base?

Answer 1

pH = 9.00

pH + pOH = 14

pOH = 5

[OH-] = 1.0 x 10^-5 M = x

B    + H2O ------------------> BH+    + OH-

0.500                                    0            0 -------------------> initial

0.500-x                                x              x ------------------> equilibrium

Kb = [BH+][OH-]/[B]

Kb = x^2 / 0.5-x

Kb = (1.0 x 10^-5 ) ^2 / 0.5                (x = neglected)

Kb = 2.0 x 10^-10