# What is the pH of a solution that has an OH^- concentration of 1*10^-9 M?

$p H = 5$
$p H + p O H = 14$
$\left[H {O}^{-}\right]$ $=$ $1 \times {10}^{-} 9$, and $p O H = - {\log}_{10} \left[H {O}^{-}\right]$ $=$ $- \left(- 9\right) = 9$
And thus $p H = 5$