The Gay-Lussac's Law of Combining Volumes states that when gases react, they do so in volumes, which are in simple ratio to one another and to the volume of the product, if any; provided temperature and pressure remain constant. It applies to only gases, which means that solid and liquid reactants and products are always ignored when applying this law. For instance, hydrogen burns in oxygen at 100°C to form steam according to the equation: 2H2(g) + O2(g) ---> 2H2O(g) 2mol 1mol 2mol 2vol 1vol 2vol 2cm^3 1cm^3 2cm^3 From the above, it implies that at 100°C, when water is in its gaseous state, 2 volumes of hydrogen gas (dm^3 or cm^3) will combine with 1 volume of oxygen gas to form 2 volumes of steam, to give a simple mole ratio of 2 : 1 : 2. Therefore, 50cm^3 of hydrogen will need 25cm^3 of oxygen to produce 50cm^3 of steam. Similarly, 15cm^3 of oxygen will require 30cm^3 of hydrogen to form 30
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