pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 500 $mL$ $HCl$ 0.9 $M$ with $V_2$ = 450 $mL$ $HClO_4$ 0.35 $M$

$HCl$ ,$ HClO_4$ :    strong acids $n_{HCl}$ $=$ $c_{HCl}\cdot V_1$ = 0.45 $mol$ $n_{HClO_4}$ $=$ $c_{HClO_4}\cdot V_2$ = 0.1575 $mol$ $pH$ $=$ $-log\frac{n_{HCl} + n_{HClO_4}}{V_1+V_2}$ = 0.194