Answer
$1.19\times 10^{-10}$
Work Step by Step
We know that
$H_3O^+=\frac{K_w}{OH^-}$
We plug in the known values to obtain:
$H_3O^+=\frac{1.00\times 10^{-14}}{8.4\times 10^{-5}}=1.19\times 10^{-10}$
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