Answer
$3.98 \times 10^{-4}$
Work Step by Step
We are given that $pH=-\log [H_3O^{+}]$ and $pH=3.4$
or, $-3.4=\log [H_3O^{+}]$
The above expression can be written as:
$[H_3O^{+}]=10^{-3.4}$
Now, we will evaluate the result by using calculator .
Therefore, our answer is: $10^{-3.4}=3.98 \times 10^{-4}$