pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 500 $mL$ $HNO_3$ 0.25 $M$ with $V_2$ = 100 $mL$ $HClO_4$ 0.05 $M$

$HNO_3$ ,$ HClO_4$ :    strong acids $n_{HNO_3}$ $=$ $c_{HNO_3}\cdot V_1$ = 0.125 $mol$ $n_{HClO_4}$ $=$ $c_{HClO_4}\cdot V_2$ = 0.005 $mol$ $pH$ $=$ $-log\frac{n_{HNO_3} + n_{HClO_4}}{V_1+V_2}$ = 0.664