Let the formula of metal chloride be MClx
Mass of 100mL of vapours of MClx at STP =0.57g
The mass of substance which occupies 22400 ml of volume at STP is molar mass(M)
∴22400100×M=0.57
∴ Molar mass of MClx=127.68gmol–1
The mass percentage of chlorine=massofthecompoundmassofchlorine×100
of Cl=127.6835.5x×100=55
⇒x=2
∴ Formula of metal chloride is MCl2