pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 300 $mL$ $HClO_4$ 0.3 $M$ with $V_2$ = 300 $mL$ $HI$ 0.3 $M$

$HClO_4$ ,$ HI$ :    strong acids $n_{HClO_4}$ $=$ $c_{HClO_4}\cdot V_1$ = 0.09 $mol$ $n_{HI}$ $=$ $c_{HI}\cdot V_2$ = 0.09 $mol$ $pH$ $=$ $-log\frac{n_{HClO_4} + n_{HI}}{V_1+V_2}$ = 0.523