Gay Lussac’s of combining volumes states that gases react in simple ratio with one another and to volumes of the products provided that temperature and pressure remain constant. In this article, you will understand how to apply this law in calculation by studying the following examples below: 1. 2H 2 + O 2 → H 2 O In the reaction above, what volume of hydrogen would be left over when 300cm 3 of oxygen and hydrogen are exploded in a sealed tube? 1cm 3 of oxygen = 2cm 3 of hydrogen 300cm 3 of oxygen = 2 x 300 = 600cm 3 Volume of left over = 1000 – 600 = 400cm 3 2. Calculate the volume of carbon (II) oxide required to react with 40cm 3 of oxygen. 2CO + O 2 → 2CO 2 1cm 3 of oxygen = 2cm 3 of CO 40cm 3 of oxygen = 2 x 40 = 80cm 3 3. Calculate the volume of residual gases that would be produced when 100cm 3 of sulphur (IV) oxide reacts with 20cm 3 of oxygen 2SO 2 + O 2 → 2SO 3 1cm 3 of O 2 = 2cm 3 20cm 3 of O 2 = 2 x 20 = 40cm 3
In this second part, we are going to be looking at the stoichiometry of reaction involving the volume of gases. Just like what we saw in mass and mole, there is a relationship which also exists between mass and volume or mole and volume that is what we are going to look at. A close look at a balanced chemical equation shows the ratio in which the reactants react and the ratio in which the products are produced. However, an experiment has shown that at standard temperature and pressure, gases have an equal volume which is numerically equal to 22.4 dm 3 or 22400cm 3 . This volume is known as the molar volume of gases. 1mole of any gas at STP = 22.4dm 3 32g of oxygen at stp = 22.4dm 3 Let us consider the decomposition of potassium trioxochlorate (V) compound. 2KClO 3(s) ---------- 2KCl (s) + 3O 2(g) 2 moles of KClO 3 will decompose to give 2 moles of KCl and 3 moles of oxygen gas. Since the molar volume gas is 22.4dm 3 . The reacting mass of KClO 3 is 2x 122.