Question

Calculate the concentrations of the species (HCN, H+, CN-and OH-)and pH in 0.65M HCN solution (K_a= 4.9 x 10^-10)

Answer 1

HCN Ka = 4.9 x 10^-10

HCN   --------------> H+ +   CN-

0.65                          0          0

0.65 - x                     x            x

Ka = [H+][CN-] / [HCN]

4.9 x 10^-10 = x^2 / 0.65 - x

x = 1.78 x 10^-5

[H+] = 1.78 x 10^-5 M

[OH-] = 5.62 x 10^-10 M

[CN-] = 1.78 x 10^-5 M

[HCN] = 0.65 M

pH = -log (1.78 x 10^-5 )

pH = 4.75