Molar mass is the mass of 1 mole of the gas.
At STP, 1 mole of gas occupies a volume of 22.4 L
5 litres of gas at STP have a mass of 12.5 grams
22.4 litres of the gas at STP have a mass of 12.5 x (22.4/5) = 56 grams
So, the molar mass of gas is 56 grams.