Answer
$4.00\times 10^{-14}M$
Work Step by Step
We know that
$[H_3O^{+}]=\frac{K_w}{OH^-}$
$[H_3O^{+}]=\frac{1.0\times 10^{-14}}{0.250}=4.00\times 10^{-14}M$
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