What is the pH?

Question

What is the pH of a 0.0780 M solution of methylamine (CH₃NH₂, Kb = 4.4 × 10⁻⁴)?

Answer 1

CH₃NH₂ + H₂O ==> CH₃NH₃⁺ + OH^-

Kb = 4.4x10^-4 = [CH₃NH₃⁺][OH^-]/[CH₃NH₂]

4.4x10^-4 = (x)(x) / 0.0780 - x and if we assume x is small relative to 0.0780 we can ignore it (see below)

4.4x10^-4 = x² / 0.0780

x² = 3.4x10^-5

x = 1.85x10^-3 M which is only 2.3% of 0.0780 so our assumption above was valid, i.e. it is less than 5%

[OH-] = 1.85x10^-3

pOH = -log 1.85x10^-3 = 2.73

pH = 14 - pOH = 14 - 2.73

pH = 11.3