 In this video we're going to learn how to calculate gas volumes for any reaction. In my previous video, Molar Volumes of Gases, you found out that one mole of any gas occupies 24 litres per mole. This is the same for all gases at room temperature and atmospheric pressure. So if we know that one mole of any gas occupies 24 litres, think 24 1 litre soft drinks bottles, then how many soft drinks bottles do you think are needed to contain two moles of any gas? Pause and choose from one of the following. The correct answer was B, 48 soft drinks bottles. This is because we doubled the number of moles of gas from one to two, and so this doubled the volume of gas produced. However, when chemical reactions take place, they don't always produce whole numbers of moles of gas, and so you need a different method to calculate the gas volume. This is where you can use a formula and ratio given from a chemical reaction. The formula is gas volume produced in litres is equal to the number of moles of gas multiplied by 24 litres per mole. So let's pose a question. In my previous video, we talked about environmental chemists wanting to know how much carbon dioxide is produced from burning a fuel. A major part of petrol is a hydrocarbon called hexane. When it is combusted in a car engine, it produces carbon dioxide, a gas and water. We're going to try and calculate the volume of carbon dioxide that is produced from this reaction. The equation tells us that for one molecule of hexane, one mole, that's six molecules or six moles of carbon dioxide produced. So this is in a ratio of one to six. Therefore, what volume of carbon dioxide do you think is produced if one mole of hexane reacts with oxygen? Pause, use the formula and choose from one of the following volumes. Continue when you are ready. The answer is C, 144 litres. This is because one mole of hexane produces six moles of carbon dioxide. So if one mole of a gas occupies 24 litres, then six moles of a gas occupy 24 litres times six, which is equal to 144 litres. However, what if you combust any amount of hexane? The amount of hexane used in mass and moles will vary according to how long the car is used. Let's say that you used the car for 20 minutes and it combusts 20.5 moles of hexane. How many moles of carbon dioxide do you think will be produced? Pause, select and continue when you are ready. The correct answer is A, because 20.5 moles times six is equal to 123 moles. Knowing this, you can calculate the gas volume using the formula to work out the volume of carbon dioxide produced. Pause, use the formula and then continue to do this. The answer is that 123 moles of carbon dioxide times 24 litres per mole is equal to 2952 litres of carbon dioxide. Did you get it right? In summary, the molar volume of any gas is always 24 litres per mole at room temperature and pressure. Using the example of Friday, you can use the ratio from any chemical equation to tell you the number of moles of gas produced from the reactants. This means you can always calculate the gas volume using the formula number of moles of gas times 24 litres per mole at room temperature and pressure.