AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Chemical equations can be interpreted in terms of moles. For example:
Full specification wording
Amounts of substances in equations (HT only): Chemical equations can be interpreted in terms of moles. For example:
A balanced symbol equation can be read as a statement about moles, not just particles.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A balanced symbol equation can be read as a statement about moles, not just particles. The coefficients tell you the number of moles of each substance that react or form. For example, 2H₂ + O₂ → 2H₂O means 2 mol of hydrogen molecules react with 1 mol of oxygen molecules to form 2 mol of water molecules. This mole interpretation allows amounts to be scaled: if 0.50 mol of O₂ reacts, then 1.0 mol of H₂ is needed and 1.0 mol of H₂O forms. The same ratio applies whatever the actual amounts, so equations can be used to calculate masses of reactants and products once amounts in moles are known.
What earns the marks
- Coefficients in a balanced equation represent the relative numbers of moles of each substance.
- The mole ratio is fixed by the equation and is independent of the actual amounts used.
- State symbols and formulae must be correct so that the mole interpretation is valid.
- The mole interpretation can be scaled up or down in direct proportion, for example doubling all amounts if one is doubled.
- Moles link to mass through Mᵣ, allowing calculation of reacting masses from the balanced equation.
- When one reactant is in excess, the limiting reactant controls the amounts of products formed.
Common mistakes and how to put them right
Reading coefficients as masses rather than moles.
Put it right: Coefficients give mole ratios; convert to mass using Mᵣ.
Ignoring coefficients when scaling.
Put it right: Use the coefficient ratio 1 : 2.
Confusing moles with molecules or atoms.
Put it right: State clearly whether the amount refers to molecules, atoms or ions.
Using an unbalanced equation.
Put it right: Balance the equation before interpreting it in moles.
Exam tips
- Underline or annotate the coefficients in the equation and write the mole ratio underneath before calculating.
- When scaling, set up a simple proportion rather than trying to remember a formula.
- Check that the equation is balanced for both atoms and charge before using it in a calculation.
- Link the mole interpretation to the quantity asked for, ensuring you convert moles to mass correctly using Mᵣ.
Try the lesson
The start of the Amounts of substances in equations (HT only): Chemical equations can be interpreted in terms of moles. For example: lesson, made from checked teaching notes for this specification point.
Have a go first
In the equation 2Mg + O₂ → 2MgO, what does the coefficient 2 in front of Mg tell you about the amounts of substances reacting?
Listen
A balanced symbol equation can be read as a statement about moles, not just particles. The coefficients tell you the number of moles of each substance that react or form. For example, 2H₂ + O₂ → 2H₂O means 2 mol of hydrogen molecules react with 1 mol of oxygen molecules to form 2 mol of water molecules. This mole interpretation allows amounts to be scaled: if 0.50 mol of O₂ reacts, then 1.0 mol of H₂ is needed and 1.0 mol of H₂O forms. The same ratio applies whatever the actual amounts, so equations can be used to calculate masses of reactants and products once amounts in moles are known. A strong answer earns marks for these points. Coefficients in a balanced equation represent the relative numbers of moles of each substance. The mole ratio is fixed by the equation and is independent of the actual amounts used. State symbols and formulae must be correct so that the mole interpretation is valid. The mole interpretation can be scaled up or down in direct proportion, for example doubling all amounts if one is doubled. A common slip: Reading coefficients as masses rather than moles: for example, treating 2H₂ as 2 g of hydrogen. Instead, coefficients give mole ratios; convert to mass using Mᵣ. Exam tip: Underline or annotate the coefficients in the equation and write the mole ratio underneath before calculating.
See it
Key words
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Balanced equations as mole ratios
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Scaling amounts in chemical reactions
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Links between moles and mass
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See every definition
- Balanced equations as mole ratios
- The coefficients tell you the number of moles of each substance that react or form.
- Scaling amounts in chemical reactions
- The same ratio applies whatever the actual amounts, so amounts can be scaled up or down in direct proportion.
- Links between moles and mass
- Equations can be used to calculate masses of reactants and products once amounts in moles are known.
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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