Calculate the pH of a 0.200 M HCN solution.

Question

Calculate the pH of a  0.200 M HCN solution. The K_a value of HCN is K_a= 6.20 ×10^–10

Answer 1

HCN ==> H⁺ + CN^-

Ka = 6.20x10^-10 = [H+][CN^-] / [HCN]

6.20x10^-10 = (x)(x) / 0.200 - x and assume x is small relative to 0.200 and ignore it in denominator

6.20x10^-10 = x² / 0.200

x² = 1.24x10^-10

x = [H⁺] = 1.11x10^-5 M

pH = -log [H⁺] = -log 1.11x10^-5

pH = 4.953