The base dissociation constant or Kb is a value used to measure the strength of a specific base in solution. To determine the percent ionization of the substance we make use of the Kb given. Methylamine or CH3NH2 when in solution would form ions:
CH3NH2 + H2O < = > CH3NH3+ + OH-
Kb is expressed as follows:
Kb = [OH-] [CH3NH3+] / [CH3NH2]
Where the terms represents the concentrations of the acid and the ions.
By the ICE table, we can calculate the equilibrium concentrations,
CH3NH2 CH3NH3+ OH-
I 0.05 0 0
C -x +x +x
--------------------------------------------------
E 0.05-x x x
Kb = [OH-] [CH3NH3+] / [CH3NH2] = 4.4 x 10^-4
4.4 x 10-4 = x^2 / 0.05-x
Solving for x,
x = [OH-] = 0.00448 M
percent ionization = 0.00448 / 0.05 x 100 = 8.96%
pH = 14 + log 0.00448 = 11.65