Answer
$K_w = [H_3O^+][OH^-]$
Work Step by Step
$K_w = \frac{[Products]}{[Reactants]}$
$H_2O(l) H_3O^+(aq) + OH^-(aq)$
$K_w = \frac{[H_3O^+][OH^-]}{[H_2O]}$
Since water is a pure liquid, we do not consider it in the constant.
$K_w = [H_3O^+][OH^-]$
