Describe how Avogadro's law can be used to compare the volumes of gases produced in different reactions, and explain why this is only valid under the same conditions of temperature and pressure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium reacts with dilute sulfuric acid to produce magnesium sulfate and hydrogen gas. Calcium carbonate reacts with dilute hydrochloric acid to produce calcium chloride, water, and carbon dioxide gas. A student wants to compare the volumes of gas produced when equal moles of each acid are used in excess with the respective solid reactants.
Model answer (4 marks)
Avogadro's law states that equal amounts of gas, expressed in moles, occupy the same volume when measured at the same temperature and pressure.
In the magnesium–sulphuric acid reaction, 1 mol of H₂ is produced per 1 mol of H₂SO₄. In the calcium carbonate–hydrochloric acid reaction, 1 mol of CO₂ is produced per 1 mol of HCl. Because the molar ratio of gas to acid is 1:1 in both reactions, the volumes of H₂ and CO₂ will be the same when the same number of moles of acid is used.
If the temperature were different, the volume of each gas would change (V ∝ T), so the comparison would no longer be valid. Likewise, if the pressure were different, the volume would change (V ∝ 1/P), again invalidating the comparison.
Therefore, Avogadro's law can be used to compare gas volumes only when the temperature and pressure are identical for all reactions.
In the magnesium–sulphuric acid reaction, 1 mol of H₂ is produced per 1 mol of H₂SO₄. In the calcium carbonate–hydrochloric acid reaction, 1 mol of CO₂ is produced per 1 mol of HCl. Because the molar ratio of gas to acid is 1:1 in both reactions, the volumes of H₂ and CO₂ will be the same when the same number of moles of acid is used.
If the temperature were different, the volume of each gas would change (V ∝ T), so the comparison would no longer be valid. Likewise, if the pressure were different, the volume would change (V ∝ 1/P), again invalidating the comparison.
Therefore, Avogadro's law can be used to compare gas volumes only when the temperature and pressure are identical for all reactions.
Examiner tips
- State Avogadro's law explicitly; mention moles and same T,P. Show the 1:1 molar ratio for each reaction. Explain why different T or P would alter volumes. Use correct terminology: molar ratio, volume, temperature, pressure.
Common mistakes
- Confusing mass with moles; writing ‘same mass of gas’ instead of ‘same moles’. Forgetting to mention that the comparison is only valid at identical temperature and pressure. Using the wrong molar ratio (e.g., 2 mol H₂ per 1 mol H₂SO₄).
Mark scheme (4 marks)
- Avogadro's law states that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure
- The molar ratio from the balanced equation determines the moles of gas produced, so reactions producing more moles of gas per mole of reactant give a greater volume
- If temperature changes, the volume of a gas changes, so comparisons would not be valid at different temperatures
- If pressure changes, the volume of a gas changes, so comparisons would not be valid at different pressures
Key terms in this question
Avogadro's law · temperature and pressure · volume
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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