AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to: calculate the volume of a gas at room…
Full specification wording
Use of amount of substance in relation to volumes of gases: Students should be able to: calculate the volume of a gas at room temperature and pressure from its mass and relative formula mass calculate volumes of gaseous reactants and products from a balanced equation and a given volume of a gaseous reactant or product change the subject of a mathematical equation.
This practical skill combines three linked calculations.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical skill combines three linked calculations. First, convert mass to moles using moles = mass ÷ relative formula mass, then find gas volume at room temperature and pressure using volume = moles × 24 dm³. For example, 8.8 g of carbon dioxide (Mᵣ = 44) gives 0.20 mol, so volume = 0.20 × 24 = 4.8 dm³. Second, use a balanced equation to set a gas volume ratio and scale it to a given volume. Third, rearrange formulas such as volume = moles × 24 dm³ to make moles, mass or relative formula mass the subject, using inverse operations and keeping units consistent. Always state the condition for 24 dm³ and convert between dm³ and cm³ where needed.
What earns the marks
- Calculates moles from mass using moles = mass ÷ relative formula mass, then multiplies by 24 dm³ to find gas volume at r.t.p.
- Uses a balanced equation to establish the volume ratio of gaseous reactants and products and scales it to the given volume.
- Rearranges a mathematical equation to change the subject, for example making moles the subject of volume = moles × 24 dm³.
- Keeps units consistent, converting between dm³ and cm³ using 1 dm³ = 1000 cm³ where necessary.
- States that the molar gas volume of 24 dm³ applies at room temperature and pressure.
Common mistakes and how to put them right
Dividing mass by volume instead of using moles = mass ÷ relative formula mass.
Put it right by calculating moles first, then using volume = moles × 24 dm³.
Rearranging an equation incorrectly, such as multiplying when division is needed.
Put it right by applying the inverse operation to both sides and checking by substitution.
Mixing units, for example using cm³ with 24 dm³.
Put it right by converting all volumes to the same unit before calculating.
Exam tips
- Show each stage: write the formula, substitute values with units, then calculate and give the unit in your answer.
- When changing the subject, perform the same inverse operation on both sides and check by substituting a simple number.
- For gas volume ratio questions, write the balanced equation and the volume ratio before doing any arithmetic.
Try the lesson
The start of the Use of amount of substance in relation to volumes of gases: Students should be able to: calculate the volume of a gas at room temperature and pressure from its mass and relative formula mass calculate volumes of gaseous reactants and products from a balanced equation and a given volume of a gaseous reactant or product change the subject of a mathematical equation. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the volume of 1.40 g of nitrogen gas (N₂) at room temperature and pressure? (Relative formula mass (Mᵣ) of N₂ = 28; molar gas volume at room temperature and pressure = 24 dm³/mol)
Listen
This practical skill combines three linked calculations. First, convert mass to moles using moles = mass ÷ relative formula mass, then find gas volume at room temperature and pressure using volume = moles × 24 dm³. For example, 8.8 g of carbon dioxide (Mᵣ = 44) gives 0.20 mol, so volume = 0.20 × 24 = 4.8 dm³. Second, use a balanced equation to set a gas volume ratio and scale it to a given volume. Third, rearrange formulas such as volume = moles × 24 dm³ to make moles, mass or relative formula mass the subject, using inverse operations and keeping units consistent. Always state the condition for 24 dm³ and convert between dm³ and cm³ where needed. A strong answer earns marks for these points. Calculates moles from mass using moles = mass ÷ relative formula mass, then multiplies by 24 dm³ to find gas volume at r.t.p. Uses a balanced equation to establish the volume ratio of gaseous reactants and products and scales it to the given volume. Rearranges a mathematical equation to change the subject, for example making moles the subject of volume = moles × 24 dm³. Keeps units consistent, converting between dm³ and cm³ using 1 dm³ = 1000 cm³ where necessary. A common slip: Dividing mass by volume instead of using moles = mass ÷ relative formula mass. Instead, calculating moles first, then using volume = moles × 24 dm³. Exam tip: Show each stage: write the formula, substitute values with units, then calculate and give the unit in your answer.
See it
Key words
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Mole calculations from mass and relative formula mass
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Molar gas volume at room temperature and pressure
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Stoichiometry from balanced equations
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Algebraic rearrangement and unit consistency
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See every definition
- Mole calculations from mass and relative formula mass
- Finding moles by dividing mass by relative formula mass, then multiplying by 24 dm³ to find gas volume at room temperature and pressure.
- Molar gas volume at room temperature and pressure
- The value of 24 dm³ that applies at room temperature and pressure and is used to find gas volume from moles.
- Stoichiometry from balanced equations
- Using a balanced equation to set a gas volume ratio and scaling it to a given volume of a gaseous reactant or product.
- Algebraic rearrangement and unit consistency
- Changing the subject of a mathematical equation using inverse operations and keeping units consistent, converting between dm³ and cm³ where needed.
Label the diagram
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