Answer
$[HI] = 3.162 \times 10^{- 3}M$
Work Step by Step
1. Calculate the hydronium concentration:
$[H_3O^+] = 10^{-pH}$
$[H_3O^+] = 10^{- 2.5}$
$[H_3O^+] = 3.162 \times 10^{- 3}M$
2. Since $HI$ is a strong acid: $[HI] = [H_3O^+] = 3.162 \times 10^{- 3}M$
