At STP 1 mole of any ideal gas has a volume of 22.4 L
So moles unknown gas = volume / 22.4 L/mol
                                    Â
= 1.20 L / 22.4 L/mol
                                    Â
= 0.05357 moles
So now we know that 0.05357 moles of gas has a mass of 1.10
g
moles = mass / molar mass
Therefore molar mass = mass / moles
molar mass = 1.10 g / 0.05357 moles
                Â
= 20.5 g/mol
Therefore, the molecular weight of unknown gas is 20.5
g/mol.
