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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 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, 10mL of nitrogen gas would react with 10 x 3 = 30mL of hydrogen gas to produce 10 x 2 = 20mL ammonia gas
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, 50mL of hydrogen gas would react with 50 x ½ = 25mL oxygen gas to produce 50mL 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)
Under Standard Laboratory Conditions (S.L.C.) of 25^oC (298K) and 101.3kPa (1 atm), 1 mole of liquid water has a volume of 18mL and will undergo electrolysis to produce 2 moles of hydrogen gas with a volume of 48.94L and 1 mole of oxygen gas with a volume of 24.47L

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