Use of amount of substance in relation to volumes of gases — AQA GCSE Chemistry
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Use of amount of substance in relation to volumes of gases explained
Use of amount of substance in relation to volumes of gases covers 3 specification points, including Equal amounts in moles of gases occupy the… and The volumes of gaseous reactants and products….
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Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.
This statement is the molar volume principle. Under identical temperature and pressure, one mole of any gas fills the same volume, because gas volume depends on the number of particles and their spacing, not on particle mass or identity. At room temperature and pressure (about 20 °C and 1 atmosphere), that volume is approximately 24 dm³, so 24 dm³ equals 24 000 cm³. Consequently, equal moles give equal volumes: 0.5 mol of any gas occupies about 12 dm³, and 2 mol occupies about 48 dm³. The relationship is volume = moles × 24 dm³ when volume is in dm³ at r.t.p. This lets you compare reacting gas volumes directly from a balanced equation, because coefficients give mole ratios and therefore volume ratios for gases measured under the same conditions.
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. 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.
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. 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.
Your focus
- State that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.
- Recall and use the molar gas volume of approximately 24 dm³ at room temperature and pressure.
- Calculate the volume of a gas from its amount in moles using volume = moles × 24 dm³ at r.t.p.
Show all 9 objectives
- Use a balanced equation to write the volume ratio of gaseous reactants and products.
- Calculate an unknown volume of a gaseous reactant or product from a given gas volume.
- Explain why coefficients in a balanced equation can be treated as volume ratios for gases under the same conditions.
- 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 gas volume.
- Change the subject of a mathematical equation used in gas volume calculations.
Use of amount of substance in relation to volumes of gases exam tips
Marking Points
- States that equal numbers of moles of any gas occupy equal volumes when temperature and pressure are the same.
- Recalls the molar gas volume at room temperature and pressure as approximately 24 dm³ per mole.
- Uses volume = moles × 24 dm³ to convert between amount in moles and gas volume at r.t.p.
- Explains that gas volume depends on number of particles and their spacing, not on the mass or identity of the gas particles.
- Applies the principle to compare volumes of different gases: equal moles give equal volumes under the same conditions.
- 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.
- 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.
Examiner Tips
- 💡Write the condition 'at room temperature and pressure' whenever you use the value 24 dm³, because the molar volume changes with temperature and pressure.
- 💡Convert mass to moles first using moles = mass ÷ relative formula mass, then multiply by 24 dm³ to find the gas volume at r.t.p.
- 💡For comparison questions, compare the number of moles of each gas rather than their masses, then state that equal moles occupy equal volumes under the same conditions.
- 💡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.
- 💡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.
Common Mistakes
- Thinking that equal masses of different gases occupy equal volumes; correct this by comparing moles, not grams, because equal moles occupy equal volumes.
- Using 24 dm³ without checking that the gas is measured at room temperature and pressure; correct this by stating the condition and converting if a different temperature or pressure is given.
- Mixing up dm³ and cm³; correct this by remembering 1 dm³ = 1000 cm³, so 24 dm³ = 24 000 cm³.
- Using the coefficients as a mass ratio instead of a volume ratio for gases; correct this 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; correct this by using only gaseous substances in the ratio.
- Forgetting to balance the equation before using coefficients; correct this by balancing the equation first so the mole and volume ratios are correct.
- Dividing mass by volume instead of using moles = mass ÷ relative formula mass; correct this by calculating moles first, then using volume = moles × 24 dm³.
- Rearranging an equation incorrectly, such as multiplying when division is needed; correct this by applying the inverse operation to both sides and checking by substitution.
- Mixing units, for example using cm³ with 24 dm³; correct this by converting all volumes to the same unit before calculating.