Question

Calculate the pH of a 0.89 M methylamine solution.

Answer 1

CH3NH2 + H2O <----------------------> CH3NH3+   + OH-

0.89                                                       0                 0 ------------------> initial

0.89-x                                                    x                  x ---------------> equilibrium

Kb = [CH3NH3+][OH-]/[CH3NH2]

Kb = x^2 / 0.89-x

4.2 x 10^-4 =x^2 / 0.89-x

x^ + 4.2 x 10^-4   x - 3.738 x 10^-4 = 0

x = 0.019

x = [OH-] = 0.019 M

pOH = -log [OH-] = -log (0.019) = 1.72

pH + pOH = 14

pH = 12.28