Answer
A cleanser has : $[OH^-] = 2.0 \times 10^{-5}M$
Work Step by Step
1. Write the $K_w$ expression for the water equilibrium. Then substitute the value for $[H_3O^+]$, and solve for $[OH^-]$:
$[H_3O^+] * [OH^-] = Kw = 10^{-14}$
$5.0 \times 10^{-10} * [OH^-] = 10^{-14}$
$[OH^-] = \frac{10^{-14}}{5.0 \times 10^{-10}}$
$[OH^-] = 2.0 \times 10^{-5}M$
