pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 100 $mL$ $HCl$ 0.65 $M$ with $V_2$ = 400 $mL$ $HNO_3$ 0.9 $M$

$HCl$ ,$ HNO_3$ :    strong acids $n_{HCl}$ $=$ $c_{HCl}\cdot V_1$ = 0.065 $mol$ $n_{HNO_3}$ $=$ $c_{HNO_3}\cdot V_2$ = 0.36 $mol$ $pH$ $=$ $-log\frac{n_{HCl} + n_{HNO_3}}{V_1+V_2}$ = 0.071