What is the pH of an aquous solution of HCN?

Question

What is the pH of a 6.18x10-3M aqueous solution of HCN? Ka for HCN = 5.8 x 10-10

Answer 1

HCN <--> H^+1 + CN^-1

Ka = [H+][CN-] / [HCN] = 5.8x10^-10

Let X = moles of HCN that ionizes to H+ and CN-

5.8x10^-10 = X^2 / (6.18x10^-3 - X)

Assume 6.18x10^-3 is very nearly equal to 6.18x10^-3 - X

X = 1.89x10^-6 M = [H+]

pH = -log[H+] = 5.72