Avogadro's law states that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure. Explain how a chemist can use Avogadro's law and a balanced chemical equation to predict the volume of gas produced in a reaction, without needing to know the mass of the gas.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
At room temperature and pressure, one mole of any gas occupies a volume of 24 dm³. A student reacts excess zinc with dilute hydrochloric acid and wants to predict the volume of hydrogen gas produced from 0.25 mol of hydrochloric acid. The equation for the reaction is: Zn + 2HCl → ZnCl₂ + H₂
Model answer (4 marks)
Avogadro's law states that equal moles of any gas occupy the same volume at the same temperature and pressure.
The balanced equation shows the mole ratio of reactants to product: 2 mol HCl give 1 mol H₂. Therefore 0.25 mol HCl will give 0.25 ÷ 2 = 0.125 mol H₂.
At room temperature and pressure the molar volume of a gas is 24 dm³ per mole. Multiply the moles of H₂ by this volume:
0.125 mol × 24 dm³ mol⁻¹ = 3 dm³.
Thus a chemist can predict that 3 dm³ of hydrogen will be produced without knowing the mass of the gas.
The balanced equation shows the mole ratio of reactants to product: 2 mol HCl give 1 mol H₂. Therefore 0.25 mol HCl will give 0.25 ÷ 2 = 0.125 mol H₂.
At room temperature and pressure the molar volume of a gas is 24 dm³ per mole. Multiply the moles of H₂ by this volume:
0.125 mol × 24 dm³ mol⁻¹ = 3 dm³.
Thus a chemist can predict that 3 dm³ of hydrogen will be produced without knowing the mass of the gas.
Examiner tips
- Use the mole ratio from the balanced equation to convert reactant moles to product moles.
- Apply the molar volume (24 dm³) to the product moles.
- Show the calculation 0.125 × 24 = 3 dm³ to demonstrate working.
Common mistakes
- Confusing the mole ratio (using 1:2 instead of 2:1).
- Using the wrong molar volume (e.g., 22.4 dm³).
- Failing to show the calculation or units.
Mark scheme (4 marks)
- Avogadro's law means equal moles of any gas occupy the same volume (at the same temperature and pressure)
- The balanced equation shows the mole ratio of reactants to gaseous product (2 mol HCl produces 1 mol H₂, so 0.25 mol HCl produces 0.125 mol H₂)
- The molar volume (24 dm³ per mole at room temperature and pressure) is multiplied by the number of moles of gas to give the volume
- The predicted volume of hydrogen produced is 3 dm³ (0.125 × 24 = 3 dm³)
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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