Amounts of substances in equations (HT only) — AQA GCSE Chemistry
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Amounts of substances in equations (HT only) explained
A balanced symbol equation shows the ratio in which particles react, and that same ratio applies to amounts in moles.
Read the full explanation
Because mass depends on Mr, the mass ratio is not simply the balancing numbers. The method is: write the balanced equation; convert the known mass to moles using n = m ÷ Mr; use the balancing numbers to find moles of the required substance; convert those moles to mass using m = n × Mr. For example, 2Mg + O₂ → 2MgO shows 2 mol Mg react with 1 mol O₂ to form 2 mol MgO. If 4.8 g Mg reacts, n(Mg) = 4.8 ÷ 24 = 0.2 mol, so n(MgO) = 0.2 mol and m(MgO) = 0.2 × 40 = 8.0 g. The equation must be balanced before any ratio is used.
Your focus
- Interpret balancing numbers as mole ratios.
- Calculate the mass of a product from the mass of a reactant.
- Calculate the mass of a reactant needed to form a given mass of product.
Amounts of substances in equations (HT only) exam tips
Marking Points
- Write or select the correctly balanced symbol equation and identify the known and required substances.
- Convert the known mass to moles using the correct Mr for that substance.
- Use the balancing numbers as a mole ratio to find the amount of the required substance.
- Convert the required amount to mass using its own Mr, not the Mr of the known substance.
- Present a clear sequence of steps with units so the reasoning can be followed.
Examiner Tips
- 💡Annotate the equation with the moles of each substance before doing arithmetic.
- 💡Keep the full calculator value until the final step, then round to a sensible number of significant figures.
- 💡Check that the total mass of reactants equals the total mass of products as a final consistency check.
Common Mistakes
- Using the mass ratio directly from the balancing numbers; correct by converting to moles first, then applying the ratio, then converting back to mass.
- Using the wrong Mr for the required product or reactant; correct by calculating Mr separately for each substance in the equation.
- Applying the ratio upside down; correct by matching the substance you know to the substance you need in the equation.