pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 50 $mL$ $HClO_4$ 0.45 $M$ with $V_2$ = 150 $mL$ $HNO_3$ 0.55 $M$

$HClO_4$ ,$ HNO_3$ :    strong acids $n_{HClO_4}$ $=$ $c_{HClO_4}\cdot V_1$ = 0.0225 $mol$ $n_{HNO_3}$ $=$ $c_{HNO_3}\cdot V_2$ = 0.0825 $mol$ $pH$ $=$ $-log\frac{n_{HClO_4} + n_{HNO_3}}{V_1+V_2}$ = 0.28