Answer
$3.16 \times 10^{-7}$
Work Step by Step
We are given that $pH=-\log [H_3O^{+}]$ and $pH=6.5$
or, $-6.5=\log [H_3O^{+}]$
The above expression can be written as:
$[H_3O^{+}]=10^{-6.5}$
Now, we will evaluate the result by using calculator .
Therefore, our answer is: $10^{-6.5}=3.16 \times 10^{-7}$