Answer
The pH of that solution is equal to 4.0
Work Step by Step
1. Calculate the pH Value
$pH = -log[H_3O^+]$
$pH = -log( 1.0 \times 10^{- 4})$
$pH = 4.0000$
2. Determine the right number of significant figures.
The $[H_3O^+]$ had 1 significant figure: $1 \times 10^{-4}$.
So, there must be only one number on the right of the decimal point of the pH.
$pH = 4.0$
