Question

A 0.100 M solution of a weak base has a pH of 9.95. What is the base hydrolysis constant, Kb, for the weak base?

Answer 1

pH + pOH = 14

pOH = 14 -9.95

pOH = 4.05

[OH-] = 10^-4.05

[OH-] = 8.9 x 10^-5 M

x = [OH-] = 8.9 x 10^-5 M

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

0.10                                          0         0   ------------------> initial

-x                                            + x         +x -----------------> changed

0.10-x                                        x           x ----------------------> equilibrium

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

Kb = x^2 / 0.1-x

Kb = (8.9 x 10^-5)^2 / (0.1-8.9 x 10^-5)

Kb = 7.93 x 10^-8