Question

The pH of a 0.10 M HCN solution is 5.20.

a. What is [H3O+ ] in that solution? _______________ M

b. What is [CN-]? What is [HCN]? (Where do the H+ and CN - ions come from?) [CN-] = _______________ M; [HCN] = _____________ M

c. What is the value of Ka for HCN? What is the value of pKa? Ka = _______________ pKa = _______________

Answer 1

HCN <------------> H⁺ + CN^-

a) we have a relation

[H3O⁺] = 10^-pH

pH = 5.20

[H3O⁺] = 10^-5.20 = 6.31 x 10^-6

[H3O⁺] = 6.31 x 10^-6 M

b) at equilibrium

[H3O⁺] = [CN^-] = 6.31 x 10^-6 M

[CN^-] = 6.31 x 10^-6 M

[HCN] = 0.10 - 6.31 x1 0^-6

[HCN] = 0.099994 M

c)   Ka = [H⁺] [CN^-] / [HCN]

Ka = [6.31 x 10^-6] [6.31 x 10^-6] / [0.099994]

Ka = 3.98 x 10^-11 / 0.099994

Ka = 3.98 x 10^-10  

pKa = - log [Ka] = - log [3.98 x 10^-10]

pKa = 9.40