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Gay-Lussac's Law of Combining Gas Volumes

Key Concepts

The volume of gases taking part in a chemical reaction show simple whole number ratios to one another when those volumes are measured at the same temperature and pressure.

Examples

  1. 1 litre of nitrogen gas reacts with 3 litres of hydrogen gas to produce 2 litres of ammonia gas

    N2(g) + 3H2(g) → 2NH3(g)

    Since all the reactants and products are gases, the mole ratio of N2(g):H2(g):NH3(g) of 1:3:2 is also the ratio of the volumes of gases

    So, 10 mL of nitrogen gas would react with 10 x 3 = 30 mL of hydrogen gas to produce 10 x 2 = 20 mL ammonia gas

  2. 2 litres of hydrogen gas react with 1 litre of oxygen gas to produce 2 litres of water vapour

    2H2(g) + O2(g) → 2H2O(g)

    Since all the reactants and products are gases, the mole ratio of H2(g):O2(g):H2O(g) of 2:1:2 is also the ratio of the volumes of gases

    So, 50 mL of hydrogen gas would react with 50 x ½ = 25 mL oxygen gas to produce 50 mL of water vapour

    When liquid water undergoes electrolysis to produce hydrogen gas and oxygen gas, the volumes of hydrogen gas and oxygen gas are produced in the ratio of 2:1 but the volume of liquid water required does not follow this relationship since the liquid water is not a gas

    2H2O(l) → 2H2(g) + O2(g)

    At 25oC (298 K) and 101.3 kPa (1 atm), 1 mole of liquid water has a volume of 18 mL and will undergo electrolysis to produce 2 moles of hydrogen gas with a volume of 48.94 L and 1 mole of oxygen gas with a volume of 24.47 L


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