pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 200 $mL$ $HI$ 0.2 $M$ with $V_2$ = 450 $mL$ $HCl$ 0.95 $M$

$HI$ ,$ HCl$ :    strong acids $n_{HI}$ $=$ $c_{HI}\cdot V_1$ = 0.04 $mol$ $n_{HCl}$ $=$ $c_{HCl}\cdot V_2$ = 0.4275 $mol$ $pH$ $=$ $-log\frac{n_{HI} + n_{HCl}}{V_1+V_2}$ = 0.143