Let the formula of metal chloride be ${\mathrm{MCl}}_{\mathrm{x}}$

Mass of $100 \mathrm{mL}$ of vapours of ${\mathrm{MCl}}_{\mathrm{x}}$ at STP $=0.57 \mathrm{g}$

The mass of substance which occupies 22400 ml of volume at STP is molar mass(M)

$\therefore \frac{100\times \mathrm{M}}{22400}=0.57$

$\therefore$ Molar mass of ${\mathrm{MCl}}_{\mathrm{x}}=127.68 \mathrm{g} {\mathrm{mol}}^{–1}$

The mass percentage of chlorine$=\frac{\mathrm{mass} \mathrm{of} \mathrm{chlorine}}{\mathrm{mass} \mathrm{of} \mathrm{the} \mathrm{compound}}\times 100$

$\%$ of $\mathrm{Cl}=\frac{35.5\mathrm{x}\times 100}{127.68}=55$

$\Rightarrow \mathrm{x}=2$

$\therefore$ Formula of metal chloride is ${\mathrm{MCl}}_2$