Determining Kb of base

Question

A 0.135 M solution of a weak base (B) has a pH of 11.23. Determine K_b for the base. B(aq) + H₂O (l) ⇄ BH⁺(aq) + OH^-(aq), K_b = ? 

Answer 1

B + H₂O ==> BH⁺ + OH^-

Kb = [BH⁺][OH^-] / [B]

From the pH, we can find the pOH and thus the [OH^-]:

pOH = 14 - pH = 14 - 11.23 = 2.77

[OH^-] = 1x10^-2.77 = 1.7x10^-3

Kb = (1.7x10^-3)(1.7x10^-3) / 0.135

Kb = 2.1x10^-5