Calculating gas volumes in reactions

You can predict the volume of a gas made in a reaction using the molar gas volume. At room temperature and pressure, 1 mole of any gas has a volume of 24 dm³. There are three steps. First, find the moles of the substance you are given, using moles = mass ÷ relative formula mass. Second, use the balanced equation to find the moles of gas from the ratio. Third, volume of gas = moles × 24.

Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Calcium carbonate has M_r = 100, so 5.0 g is 5.0 ÷ 100 = 0.050 mol. One mole of calcium carbonate gives one mole of carbon dioxide, so 0.050 mol of CO₂ is made. Volume = 0.050 × 24 = 1.2 dm³, which is 1200 cm³.

Always check the ratio. Hydrogen peroxide decomposes by 2H₂O₂ → 2H₂O + O₂. Two moles of hydrogen peroxide give only one mole of oxygen, so 0.20 mol of hydrogen peroxide gives 0.10 mol of oxygen. The volume is 0.10 × 24 = 2.4 dm³ at room temperature and pressure.