Calculate the Kb

Question

1. A 0.135 M solution of a weak base has a pH of 11.23. Determine Kb for the base at 298 Kelvin.

Answer 1

Let the weak base be represented by B.

B + H2O ==> BH⁺ + OH- .. hydrolysis of B

Kb = [BH⁺][OH-] / [B]

From the pH, we can determine pOH and then [OH-]. Since [OH-] = [BH⁺] and we are given [B], we can find the Kb

pH = 11.23

pOH = 14 - 11.23 = 2.77

[OH-] = 1x10^-2.77 = 1.70x10^-3 M

Kb = (1.70x10^-3)(1.70x10^-3) / 0.135

Kb = 2.14x10^-5