Ionization Constants

Question

Determine the relative acid strengths of NH4+NH4+ and HCN by comparing their ionization constants. The ionization constant of HCN is 4.9 ×10⁻¹⁰. For the ionization constant of NH4+, the ionization constant of its conjugate base, NH₃, is listed as 1.8 ×× 10⁻⁵.

Answer 1

NH₄⁺ + H2O ==> H3O⁺ + NH3

Ka(NH₄⁺) = Kw/Kb = 1x10^-14 / 1.8x10^-5 = 5.56x10^-10

HCN + H2O ==> H3O + CN^-

Ka = 4.9x10^-10

Comparing 5.56x10^-10 to 4.9x10^-10 , we would say that NH4⁺ is every so slightly a stronger acid than HCN, but for all practical purposes, they seem to have the same strength.