## Chemistry: Molecular Approach (4th Edition)

The $[OH^-]$ of a solution decreases when the $[H_3O^+]$ is increased, and increases when the $[H_3O^+]$ is decreased.
According to the expression of the ion product constant for water: $$K_w = [H_3O^+][OH^-]$$ - Dividing both sides by $[H_3O^+]$: $$\frac{K_w}{[H_3O^+]} = [OH^-]$$ - $K_w$ is a constant, assuming constant temperature; therefore, $[OH^-]$ is inversely proportional to $[H_3O^+]$. $$\frac{1}{[H_3O^+]} \propto [OH^-]$$ When $[H_3O^+]$ increases, $[OH^-]$ decreases and vice versa.