Avogadro's law says that equal volumes of different gases, at the same temperature and pressure, contain the same number of molecules. It follows that the volume of a gas is directly proportional to the number of moles of gas.
This gives a shortcut for reactions where all the gases are measured at the same temperature and pressure. The ratio of the volumes of the gases is the same as the ratio of the moles in the balanced equation. You do not need to find the Mr or the moles, and you do not need the molar volume at all.
Example: 2H2 + O2 → 2H2O (steam). The ratio is 2 : 1 : 2, so 20 cm3 of hydrogen reacts with 10 cm3 of oxygen and makes 20 cm3 of steam. Another example: CH4 + 2O2 → CO2 + 2H2O. Burning 50 cm3 of methane needs 2 x 50 = 100 cm3 of oxygen and gives 50 cm3 of carbon dioxide. If the water is cooled to a liquid, it is no longer a gas, so it is not counted in the gas volume.
