Textbook Problem

The weak base methylamine, ${\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}$, has $K_{\text{b}}\text{= 4}\text{.2}\times {\text{10}}^{-\text{4}}$. It reacts with water according to the equation.

${\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}{\text{(aq) + H}}_{\text{2}}\text{O(}\mathcal{l}\text{) }\rightleftharpoons {\text{CH}}_{\text{3}}{\text{NH}}_3^{+}{\text{(aq) + OH}}^{-}(\text{aq})$

Calculate the equilibrium hydroxide ion concentration in a 0.25 M solution of the base. What are the pH and pOH of the solution?