$a A(g)+b B(g) \rightarrow c C(g)+d D(g)$

Here, '$a$' moles of $A(g)$ reacts with '$b$' moles of $B(g)$ to give '$c$' mole of $C(g)$ and '$d$' moles of $D(g)$

No. of moles of $\mathrm{AgNO}_{3}=10^{-3} \mathrm{mol}$

No. of moles the chloride $=0.5 \times 10^{-3} \mathrm{mol}$

Suppose the formula for the chloride is $\mathrm{XCl}_{\mathrm{n}}$ then moles of chloride ion $=\mathrm{n} \times 0.5 \times 10^{-3}$

Reaction goes as follows:

$A g^{+}+C l^{-} \rightarrow A g C l$

Then, going by stoichiometry we get

$n \times 0.5 \times 10^{-3}=10^{-3}$

$\Rightarrow n=2$

Therefore, formula is $\mathrm{XCl}_{2}$