AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The volumes of gaseous reactants and products can be calculated from the…
Full specification wording
Use of amount of substance in relation to volumes of gases: The volumes of gaseous reactants and products can be calculated from the balanced equation for the reaction.
A balanced equation gives the mole ratio of gaseous reactants and products.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A balanced equation gives the mole ratio of gaseous reactants and products. Because equal moles of gases occupy the same volume under the same conditions, the coefficients also give the volume ratio for gases measured under identical conditions. For example, in 2H₂(g) + O₂(g) → 2H₂O(g), two volumes of hydrogen react with one volume of oxygen to form two volumes of water vapour, if all volumes are measured at the same temperature and pressure. If 20 cm³ of hydrogen reacts completely, it needs 10 cm³ of oxygen and produces 20 cm³ of water vapour. The method is: write the balanced equation, use coefficients to set the volume ratio, then scale to the given volume. Only gases are included in the volume ratio; solids and liquids are not.
What earns the marks
- Writes or uses the balanced equation to identify the mole ratio of gaseous reactants and products.
- States that coefficients in the balanced equation give the volume ratio for gases measured under the same conditions of temperature and pressure.
- Scales the volume ratio to the given volume of a gaseous reactant or product to calculate the required volume.
- Excludes solids and liquids from the gas volume ratio because the molar volume principle applies only to gases.
- Checks that all gas volumes are compared at the same temperature and pressure before applying the ratio.
Common mistakes and how to put them right
Using the coefficients as a mass ratio instead of a volume ratio for gases.
Put it right by applying coefficients to volumes only when all substances compared are gases at the same conditions.
Including a solid or liquid in the volume ratio.
Put it right by using only gaseous substances in the ratio.
Forgetting to balance the equation before using coefficients.
Put it right by balancing the equation first so the mole and volume ratios are correct.
Exam tips
- Write the balanced equation and label the state symbols so you can see which substances are gases.
- Underline or list the coefficients of the gaseous substances, then write them as a volume ratio before substituting the given volume.
- Check that the question states the same temperature and pressure for all gas volumes; if it does not, do not assume the simple volume ratio applies.
Try the lesson
The start of the Use of amount of substance in relation to volumes of gases: The volumes of gaseous reactants and products can be calculated from the balanced equation for the reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
In the equation N₂(g) + 3H₂(g) → 2NH₃(g), what volume of ammonia is produced from 1 dm³ of nitrogen if all volumes are measured under the same conditions?
Listen
A balanced equation gives the mole ratio of gaseous reactants and products. Because equal moles of gases occupy the same volume under the same conditions, the coefficients also give the volume ratio for gases measured under identical conditions. For example, in 2H₂(g) + O₂(g) → 2H₂O(g), two volumes of hydrogen react with one volume of oxygen to form two volumes of water vapour, if all volumes are measured at the same temperature and pressure. If 20 cm³ of hydrogen reacts completely, it needs 10 cm³ of oxygen and produces 20 cm³ of water vapour. The method is: write the balanced equation, use coefficients to set the volume ratio, then scale to the given volume. Only gases are included in the volume ratio; solids and liquids are not. A strong answer earns marks for these points. Writes or uses the balanced equation to identify the mole ratio of gaseous reactants and products. States that coefficients in the balanced equation give the volume ratio for gases measured under the same conditions of temperature and pressure. Scales the volume ratio to the given volume of a gaseous reactant or product to calculate the required volume. Excludes solids and liquids from the gas volume ratio because the molar volume principle applies only to gases. A common slip: Using the coefficients as a mass ratio instead of a volume ratio for gases. Instead, applying coefficients to volumes only when all substances compared are gases at the same conditions. Exam tip: Write the balanced equation and label the state symbols so you can see which substances are gases.
See it
Put it in order
Put these steps in the order they happen.
- 1. Include only gases in the volume ratio
- 2. Use coefficients to set the volume ratio
- 3. Write the balanced equation
- 4. Calculate the required gas volume
- 5. Scale the volume ratio to the given volume
- 6. Check all gas volumes at the same temperature and pressure
Key words
Say what each word means before you turn the card.
Balanced equations and mole ratios
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Volume ratios of gases
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Stoichiometry of gaseous reactions
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Same conditions of temperature and pressure
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See every definition
- Balanced equations and mole ratios
- A balanced equation gives the mole ratio of gaseous reactants and products, and its coefficients can be used to set the volume ratio.
- Volume ratios of gases
- The coefficients in the balanced equation give the volume ratio for gases measured under the same conditions of temperature and pressure.
- Stoichiometry of gaseous reactions
- The volumes of gaseous reactants and products can be calculated from the balanced equation by scaling the volume ratio to the given volume.
- Same conditions of temperature and pressure
- Equal moles of gases occupy the same volume under the same conditions, so gas volumes are compared only at the same temperature and pressure.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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