5.6 litres of gas Z weighs 32 g at STP. What is the molecular weight and vapour density of Z ?

At STP, 22.4 L of any gas = 1 mole

∴ Number of moles of Z = $\frac{5.6}{22.4}$ = 0.25 mole

Molecular weight of Z = $\frac{\text{Mass}}{\text{Number of moles}}$ = $\frac{32}{0.25}$ = 128 g/mol

Vapour density = $\frac{\text{Molecular weight}}{2}$ = $\frac{128}{2}$ = 64

Hence, the molecular weight of Z is 128 and its vapour density is 64.