Amounts of substances in equations (HT only) — AQA GCSE Chemistry
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Amounts of substances in equations (HT only) explained
Amounts of substances in equations (HT only) covers 2 specification points, including The masses of reactants and products can be… and Calculate the masses of substances shown in a….
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The masses of reactants and products can be calculated from balanced symbol equations.
A balanced symbol equation shows the ratio in which particles react, and that same ratio applies to amounts in moles. 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.
Students should be able to: calculate the masses of substances shown in a balanced symbol equation calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.
This skill has two parts. First, from a balanced equation alone, calculate the mass of each substance shown by treating the balancing numbers as moles and multiplying each by its Mr. For 2H₂ + O₂ → 2H₂O, 2 mol H₂ has mass 4 g, 1 mol O₂ has mass 32 g and 2 mol H₂O has mass 36 g, so 4 g + 32 g = 36 g, confirming conservation of mass. Second, when a mass of one reactant or product is given, convert it to moles, apply the mole ratio from the equation, then convert to the mass of the required substance. For example, if 8 g of H₂ reacts, n(H₂) = 8 ÷ 2 = 4 mol, so n(H₂O) = 4 mol and m(H₂O) = 4 × 18 = 72 g. Always use the Mr of the substance being converted.
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.
Show all 6 objectives
- Calculate the mass of every substance represented in a balanced equation.
- Use a given mass of one substance to calculate masses of other reactants or products.
- Verify conservation of mass in a balanced equation calculation.
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.
- Calculate the mass of each substance in an equation by multiplying its balancing number by its Mr.
- Show that the total mass of reactants equals the total mass of products for a fully balanced equation.
- For a given mass, convert to moles using the correct Mr before applying the mole ratio.
- Apply the balancing-number ratio correctly and convert the result to mass using the Mr of the required substance.
- Give answers with correct units and appropriate significant figures.
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.
- 💡Label each substance in the equation with its moles and mass to organise the calculation.
- 💡For the first part, check that reactant masses sum to product masses; this catches arithmetic slips.
- 💡For the second part, write the three stages clearly: moles known, ratio, moles required, then mass required.
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.
- Treating balancing numbers as masses; correct by multiplying each balancing number by that substance's Mr.
- Forgetting to convert the given mass to moles before using the ratio; correct by always starting with n = m ÷ Mr.
- Using the Mr of the known substance for the required substance; correct by calculating the required substance's Mr separately.