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.