Gas volumes – GCSE Combined Science Chemistry

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Gases fill balloons, power fizzy drinks and escape from reactions. You cannot weigh a gas easily, but you can measure its volume. That is how much space it takes up.

This matters because many reactions make or use gases. Chemists need to predict how much gas they will get. They can do that with one simple number.

By the end you will be able to convert between moles and gas volume. You will find the volume of a gas made from a given mass. You will also use the big numbers in an equation to compare gas volumes.

Equal volumes, equal molecules

Equal volumes of different gases at the same temperature and pressure contain the same number of molecules. Picture two balloons of the same size. One holds hydrogen and one holds carbon dioxide. They hold the same number of molecules, though the carbon dioxide has more mass.

This works because the particles in a gas are very far apart. Imagine a school hall with a few people spread out. It does not matter whether they are tall or short, because there is so much empty space.

The molar volume

A mole is a fixed number of particles. The volume of one mole of any gas at room temperature (20 °C) and pressure (1 atmosphere) is 24 dm³. This is also 24 000 cm³.

It does not matter which gas it is. One mole of hydrogen takes up 24 dm³. So does one mole of oxygen or carbon dioxide.

This is the number that links moles to volume. It is called the molar volume.

Moles and volume

One mole takes up 24 dm³, so two moles take up twice that. To find a volume, multiply by 24. volume of gas (dm³) = moles × 24. To find moles, divide. moles = volume (dm³) ÷ 24.

If the volume is in cm³, use 24 000 instead of 24. For example, 600 cm³ is 600 ÷ 24 000 = 0.025 mol.

Check the units before you start. A dm³ is 1000 cm³. Mixing them up is an easy slip.

From mass to gas volume

Often you are given a mass, not moles. Use three steps. First turn the mass into moles. Then use the equation to find the moles of gas. Finally multiply by 24.

Take CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Find the volume of carbon dioxide from 10 g of calcium carbonate, Mr = 100.

Moles of CaCO₃ = 10 ÷ 100 = 0.10 mol. The ratio of CaCO₃ to CO₂ is 1 : 1, so 0.10 mol of CO₂ forms. Volume = 0.10 × 24 = 2.4 dm³.

Reacting volumes of gases

Equal volumes hold equal numbers of molecules. So the ratio of gas volumes in a reaction matches the ratio of the big numbers in the balanced equation. You do not need moles at all.

N₂ + 3H₂ → 2NH₃ shows 1 volume of nitrogen reacts with 3 volumes of hydrogen. It makes 2 volumes of ammonia. So 30 dm³ of nitrogen reacts with 90 dm³ of hydrogen to make 60 dm³ of ammonia.

All the gases must be measured at the same temperature and pressure.

When one gas is in excess

In 2H₂ + O₂ → 2H₂O, the ratio of H₂ to O₂ is 2 : 1. So 100 cm³ of hydrogen needs 50 cm³ of oxygen. It makes 100 cm³ of steam.

A gas is in excess if there is more than enough. Some is left over at the end. It is like having ten slices of bread but only four slices of cheese.

Say 100 cm³ of hydrogen and 100 cm³ of oxygen react. Only 50 cm³ of oxygen reacts, so 50 cm³ of oxygen is left over.

Recap

Equal volumes of gases at the same temperature and pressure hold equal numbers of molecules.

One mole of any gas is 24 dm³ at room temperature and pressure. Use 24 000 for cm³.

For mass to volume, go mass, then moles, then use the ratio, then multiply by 24.

Gas volumes react in the same ratio as the big numbers in the equation.