What is the pH?

Question

What is the pH of a 0.0580 M solution of hydrocyanic acid, HCN (Ka = 4.9 × 10⁻¹⁰)?

Answer 1

HCN <==> H⁺ + CN^-

Ka = [H⁺][CN^-] / [HCN]

4.9x10^-10 = (x)(x) / 0.0580 - x (if we assume x is small relative to 0.058 we can ignore it)

4.9x10^-10 = x² / 0.0580

x² = 2.84x10^-11

x = 5.33x10^-6 = [H⁺] (note: this value is quite small relative to 0.058 so above assumption was valid)

pH = -log [H⁺] = -log 5.33x10^-6

pH = 5.27