Amount of Substance in Relation to Volumes of Gases (OCR GCSE Chemistry A (Gateway Science Suite)): Revision Notes

At room temperature (20°C) and standard pressure (1 atm), one mole of any gas occupies a volume of 24 dm³ (or 24,000 cm³). This is known as the molar gas volume. This relationship is useful because if you know the number of moles of a gas, you can easily calculate its volume, and vice versa.

To find the volume of a gas in dm³, use the formula:

• If you need the volume in cm³, multiply the volume in dm³ by 1,000.

Worked Example 1

• Problem: Calculate the volume of 7.52 g of ammonia $(NH₃)$gas at room temperature and pressure.

1. Calculate moles of NH₃:

2. Calculate volume:

Worked Example 2

• Problem: 3.25 g of zinc (Zn) reacts with excess hydrochloric acid $(HCl)$ to form zinc chloride $(ZnCl₂)$and hydrogen gas $(H₂)$. Calculate the volume of hydrogen gas produced.

3. Calculate moles of zinc:

4. Use mole ratio from the balanced equation: Since the ratio of $Zn$ to $H₂$ is 1:1, the moles of hydrogen gas produced will also be 0.05 mol.
5. Calculate the volume of hydrogen gas:

6. Convert to cm³: