Q.Calculate number of moles of hydrogen in 0.448 litre of hydrogen gas at STP (Ans. : 0.02 mol)

Because it is often far more convenient to measure the VOLUME of a gas than to weigh its mass, chemists rely on a consequence of Avogadro's law: at a given temperature and pressure, one mole of ANY gas — regardless of its identity — occupies exactly the same volume. At standard temperature and pressure (STP, historically taken as 0°C and 1 atmosphere), this molar volume works out to 22.4 dm3 (22.4 litres) per mole. (IUPAC's more recent STP convention, using 1 bar of pressure instead of 1 atmosphere, gives a very slightly different molar volume of 22.71 L/mol.) Once a gas's volume at STP is known, its number of moles follows immediately from dividing that volume by the molar volume, with no need to know the gas's mass or molar mass at all.