Mass of gas = 12 g
Volume of 12 g of gas at STP =5.6 L
22.4 L of gas at STP =1 mole
5.6 L of gas at STP =5.622.4moles =0.25 moles
mass of 0.25 moles of gas =12 g
Mass of 1 mole of gas or molar mass =12/0.25 = 48 g/mol
Relative molecular mass of gas =48 u