Answer
Please see the work below.
Work Step by Step
We know that
$H_3O^+=\frac{K_w}{OH^-}$
$H_3O^+=\frac{1.0\times 10^{-14}}{0.050}=2\times 10^{-3}M$
$PH=-log(H_3O^+)$
$PH=-log (2\times 10^{-13})=12.698$
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