[OH-], pH, and pOH of ammonia solution

Question

Determine the [OH^-], pH, and pOH of a 0.15 M ammonia solution. NH₃(aq) + H₂O(l)  ⇄ NH₄⁺(aq) + OH^-(aq)

Answer 1

NH3 is a weak base so we need to know the Kb for NH₃ or the Ka for the conjugate acid, NH₄⁺ before we can appropriately answer this question.

Kb NH3 = 1.8x10^-5

Kb = [NH4+][OH-]/[NH3]

1.8x10^-5 = (x)(x)/0.15 - x and assuming x is small relative to 0.15, we can neglect it in the denominator

x² = 2.7x10^-6

x = [OH-] = 1.6x10^-3 (this is small compared to 0.15 so assumption was valid)

pOH = -log [OH-] = 2.78

pH = 14 - pOH = 11.2