Answer
Please see the work below.
Work Step by Step
We know that
$H_3O^+=\frac{K_w}{OH^-}$
$H_3O^+=\frac{1.0\times 10^{-14}}{0.0040}=2.5\times 10^{-12}M$
Now $PH=-log (H_3O^+)=-log(2.5\times 10^{-12})=11.602$
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