HCN is a weak acid (Ka = 6.20 x 10-10), so the salt, KCN, acts as a weak base. What is the pH of a solution that is 0.0315 M in KCN at 25 °C?

Answer 1

KCN is the salt of a strong base (KOH) and a weak acid (HCN). Thus, the pH of the solution should be >7 (basic). To find the pH of 0.0315 M KCN solution, we need to look at the hydrolysis of CN^-:

CN^- + H2O ==> HCN + OH^-

Kb = [HCN][OH-] / [CN-]

Kb = Kw/Ka = 1x10^-14 / 6.20x10^-10 = 1.61x10^-5

1.61x10^-5 = [x][x] / 0.0315 - x (assume x is small relative to 0.0315 and ignore it in denominator)

1.61x10^-5 = [x][x] / 0.0315

x² = 5.08x10^-7

x = 7.13x10^-4 M = [OH-]

pOH = -log [OH-] = -log 7.13x10^-4 = 3.15

pH = 14 - pOH = 14 - 3.15

pH = 10.9 (basic as predicted)