What is the pH?

Question

What is the pH of a 0.0870 M solution of methylamine (CH₃NH₂, Kb = 4.4 × 10⁻⁴)?

Answer 1

CH₃NH₂ + H₂O ==> CH₃NH₃⁺ + OH^-

Kb = [CH₃NH₃⁺][OH^-] / [CH₃NH₂]

4.4x10^-4 = (x)(x) / 0.0870 - x

Use the quadratic formula and solve for x

x = 0.00597 M = [OH^-]

pOH = -log 0.00597

pOH = 2.22

pH = 14 - 2.22

pH = 11.78