AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Calculate the masses of reactants and products from the balanced symbol…
Full specification wording
Amounts of substances in equations (HT only): calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.
This Higher Tier skill uses a balanced symbol equation to link the mass of one substance to the mass of another.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier skill uses a balanced symbol equation to link the mass of one substance to the mass of another. The balancing numbers give the ratio of amounts in moles. First convert the given mass to moles by dividing by the relative formula mass (Mr). Then use the ratio from the equation to find the moles of the required substance. Finally multiply by its Mr to get the mass. For example, in 2Mg + O₂ → 2MgO, if 4.8 g of Mg reacts, moles of Mg = 4.8 ÷ 24 = 0.20 mol. The ratio Mg:MgO is 2:2, so 0.20 mol MgO forms. Mass MgO = 0.20 × 40 = 8.0 g. This method works for any reactant or product once the equation is balanced.
What earns the marks
- Convert the given mass to moles using moles = mass ÷ Mr, with Mr calculated from the correct formula.
- Use the balancing numbers in the balanced symbol equation to find the mole ratio between the given substance and the required substance.
- Calculate the moles of the required substance by multiplying the moles of the given substance by the ratio of its balancing number to the given substance's balancing number.
- Convert the moles of the required substance to mass using mass = moles × Mr.
- Show clear working, including the balanced equation, the mole ratio and the final unit (g).
Common mistakes and how to put them right
Using the balancing numbers as a mass ratio instead of a mole ratio.
Put it right: Balancing numbers give the ratio of amounts in moles, so convert masses to moles first.
Forgetting to multiply by the balancing number ratio, for example assuming a 1:1 mole ratio when the equation shows 2:3.
Put it right: Write the ratio from the balanced equation before calculating.
Using the wrong Mr, such as using the Mr of the given substance for the required substance.
Put it right: Calculate the Mr of each substance separately from its formula.
Exam tips
- Write the balanced equation at the start of your answer so the mole ratio is clear.
- Label each step with its quantity and unit, for example 'moles of Mg = 4.8 ÷ 24 = 0.20 mol', to make your method easy to follow.
- Check that your final answer is in grams and has a sensible magnitude compared with the given mass.
Try the lesson
The start of the Amounts of substances in equations (HT only): calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product. lesson, made from checked teaching notes for this specification point.
Have a go first
Calcium carbonate decomposes on heating: CaCO₃ → CaO + CO₂. What mass of calcium oxide is produced when 25 g of calcium carbonate decomposes completely? (Mᵣ: CaCO₃ = 100, CaO = 56, CO₂ = 44)
Listen
This Higher Tier skill uses a balanced symbol equation to link the mass of one substance to the mass of another. The balancing numbers give the ratio of amounts in moles. First convert the given mass to moles by dividing by the relative formula mass (Mr). Then use the ratio from the equation to find the moles of the required substance. Finally multiply by its Mr to get the mass. For example, in 2Mg + O₂ → 2MgO, if 4.8 g of Mg reacts, moles of Mg = 4.8 ÷ 24 = 0.20 mol. The ratio Mg:MgO is 2:2, so 0.20 mol MgO forms. Mass MgO = 0.20 × 40 = 8.0 g. This method works for any reactant or product once the equation is balanced. A strong answer earns marks for these points. Convert the given mass to moles using moles = mass ÷ Mr, with Mr calculated from the correct formula. Use the balancing numbers in the balanced symbol equation to find the mole ratio between the given substance and the required substance. Calculate the moles of the required substance by multiplying the moles of the given substance by the ratio of its balancing number to the given substance's balancing number. Convert the moles of the required substance to mass using mass = moles × Mr. A common slip: Using the balancing numbers as a mass ratio instead of a mole ratio. Instead, balancing numbers give the ratio of amounts in moles, so convert masses to moles first. Exam tip: Write the balanced equation at the start of your answer so the mole ratio is clear.
Key words
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Mole ratio
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Relative formula mass (Mr)
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Moles
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Balanced symbol equation
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See every definition
- Mole ratio
- The ratio of amounts in moles given by the balancing numbers in the balanced symbol equation.
- Relative formula mass (Mr)
- A value calculated from the correct formula, used to convert between mass and moles.
- Moles
- An amount found by dividing the given mass by the relative formula mass.
- Balanced symbol equation
- An equation whose balancing numbers give the ratio of amounts in moles.
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