Question

The ph of a 1.0 M solution of the weak acid HCN is 4.65. What is the ionization constant of this acid?

Answer 1

balanced chemical equation for the ionization of the weak acid (HCN)-

HCN + H2O ----------> H3O+ + CN-

Since, [H3O+] = 10^-pH

pH = 4.65

[H3O+] = 10^(-4.65)

Since according to Reaction [H3O+] = [ CN-]

.......HCN + H2O ----------> H3O+ + CN-

Initial 1.0M .......................... 0 M 0

Eq. (1- 10^-4.65) ........10^-4.65 10^-4.65

Ionisation constant Ka = [H3O+] [ CN-] / [HCN]

Ka = 10^(-4.65) * 10^(-4.65) / (1.0 - 10^(-4.65))

Since, 10^(-4.65) is much smaller than 1 , so it can be neglected. (1.0 - 10^(-4.65)) ≈ 1.0

Ka = 10^(-4.65) * 10^(-4.65) / 1.0

Ka = 1.0 * 10^(-9.2)