# What is the hydronium ion concentration of a solution that has a pH of 8.57?

$\left[{H}_{3} {O}^{+}\right] < {10}^{-} 7 \cdot m o l \cdot {L}^{-} 1$. The solution is assumed to be aqueous.
By definition, $p H$ $=$ $- {\log}_{10} \left[{H}_{3} {O}^{+}\right]$
And thus $\left[{H}_{3} {O}^{+}\right]$ $=$ ${10}^{- 8.57} \cdot m o l \cdot {L}^{-} 1$ $=$ ??*mol*L^-1.
What is $\left[H {O}^{-}\right]$ for this solution?