400 ml of a gas (@stp) has a mass of 0.792 g. what is the molecular mass of this gas

Answer is: molar mass is 44 g/mol.

m (gas) = 0,792g.

V (gas) = 400 ml = 400 cm³ = 0,4 dm³.

n (gas) = m (gas) : M (gas)

n (gas) = V (gas) : Vm.

Vm - molar volume, on STP Vm = 22,4 dm³/mol.

n (gas) = 0,4 dm³ : 22,4 dm³/mol = 0,018 mol.

M (gas) = m : n = 0,792 g : 0,018 mol

M (gas) = 44 g/mol.