Question

Calculate the initial concentration of an aqueous solution of the weak acid, hydrocyanic acid (HCN), with a pH of 6.4. The pKa of hydrocyanic acid is 9.31.

Answer 1

pKa = 9.31

pH = 6.4

[H+] = 10^-6.4 = 3.981 x 10^-7 M

Ka = 4.90 x 10^-10

HCN   --------------> H+   + CN-

C                             0         0

C - x                          x          x

Ka = x^2 / C - x

x = [H+] = 3.981 x 10^-7 M

4.90 x 10^-10 = (3.981 x 10^-7)^2 / C - 3.981 x 10^-7

C = 3.24 x 10^-4

initial concentration of acid = 3.24 x 10^-4 M