Equal volumes of different gases at the same temperature and pressure contain the same number of molecules. This means one mole of any gas takes up the same volume as one mole of any other gas, whatever the gas is. At room temperature and pressure (RTP, 20 °C and 1 atmosphere) this molar gas volume is 24 dm3. So one mole of hydrogen, one mole of oxygen and one mole of carbon dioxide each occupy 24 dm3 at RTP.
This gives two relationships. Volume of gas (dm3) = moles × 24, and moles = volume (dm3) ÷ 24. For example, 12 dm3 of a gas is 0.5 mol, and 0.25 mol of a gas has a volume of 6 dm3.
Gas volumes in experiments are measured by collecting the gas in a gas syringe, or in an upturned measuring cylinder full of water. These usually read in cm3, so divide by 1000 to get dm3. For example, a gas syringe collects 480 cm3 of hydrogen. This is 0.48 dm3, which is 0.48 ÷ 24 = 0.020 mol.