Amounts of substances in equations (HT only) — AQA GCSE Combined Science
Test yourself on Amounts of substances in equations (HT only) with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Amounts of substances in equations (HT only) explained
A balanced symbol equation fixes the ratio in which particles react, so it also fixes the ratio of amounts in moles.
Read the full explanation
To calculate masses, first balance the equation, then read the coefficients as the number of moles of each substance. Convert any given mass to moles using n = m ÷ Mᵣ, apply the mole ratio from the coefficients, and convert back to mass using m = n × Mᵣ. For example, in 2Mg + O₂ → 2MgO, burning 4.8 g of Mg (Mᵣ = 24) gives n(Mg) = 4.8 ÷ 24 = 0.20 mol; the ratio Mg : MgO is 2 : 2, so n(MgO) = 0.20 mol and m(MgO) = 0.20 × 40 = 8.0 g. The same method works for any reactant or product once the equation is balanced.
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. 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.
Mg + 2HCl → MgCl₂ + H₂
This balanced equation is a quantitative recipe for the reaction between magnesium and hydrochloric acid. The large numbers in front of formulae are stoichiometric coefficients: they tell you the ratio in which particles react and are produced. One Mg atom reacts with two HCl molecules, giving one MgCl₂ formula unit and one H₂ molecule. The small subscript ₂ in H₂ shows atoms within a particle, not amounts that can be changed. To use the equation, convert masses to moles, apply the coefficients as a ratio, then convert back. For example, 2.4 g Mg is 0.10 mol, so it needs 0.20 mol HCl and forms 0.10 mol H₂, which has a mass of 0.20 g.
shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.
The coefficients in a balanced equation give the mole ratio for the reaction. In Mg + 2HCl → MgCl₂ + H₂, one mole of magnesium atoms reacts with two moles of hydrochloric acid molecules. This produces one mole of magnesium chloride formula units and one mole of hydrogen molecules. The ratio is 1:2:1:1. Use it by converting any given mass to moles, multiplying or dividing by the ratio, then converting to the requested mass. For example, 0.25 mol Mg needs 0.50 mol HCl and gives 0.25 mol H₂, which has a mass of 0.50 g. This fundamental principle allows chemists to predict the exact mass of product formed or reactant required, ensuring no materials are wasted.
calculate the masses of substances shown in a balanced symbol equation
This statement requires you to calculate masses of substances from a balanced symbol equation. Note that this specific quantitative skill is strictly for Higher Tier (HT) candidates. The method involves three main stages: write the balanced equation; convert the known mass to moles using moles = mass ÷ relative formula mass; use the stoichiometric coefficients to find the moles of the unknown substance; then convert back to mass using mass = moles × relative formula mass. For example, in 2H₂ + O₂ → 2H₂O, 4 g of H₂ is 2 mol, which needs 1 mol of O₂ (32 g) and forms 2 mol of H₂O (36 g). The calculated masses must always be consistent with the law of conservation of mass, meaning the total reactant mass equals the total product mass.
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. 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.
Your focus
- Balance a symbol equation and state the mole ratio of reactants and products.
- Convert between mass and moles using n = m ÷ Mᵣ and m = n × Mᵣ.
- Use a balanced equation to calculate the mass of a reactant or product from a given mass.
Show all 18 objectives
- State the mole ratio from a balanced symbol equation.
- Explain what the coefficients in an equation represent in terms of moles.
- Use a mole ratio to calculate the amount of a reactant or product from a given amount in moles.
- Interpret the coefficients in Mg + 2HCl → MgCl₂ + H₂ as a mole ratio.
- Convert between mass and moles using the equation.
- Distinguish coefficients from subscripts when balancing and using equations.
- Use the coefficients in a balanced equation to state mole ratios.
- Perform a multi-step calculation from a given mass to moles of another reactant or product.
- Explain why mole ratios, not mass ratios, are taken directly from an equation.
- Calculate relative formula masses from a given chemical formula.
- Use a balanced symbol equation to convert between masses of different substances.
- Verify a mass calculation by checking the conservation of mass.
- Convert a given mass of a reactant or product into moles using its relative formula mass.
- Apply the mole ratio from a balanced symbol equation to find the moles of another substance.
- Convert the moles of the required substance into its mass in grams.
Amounts of substances in equations (HT only) exam tips
Marking Points
- A correct balanced symbol equation is required before any mass calculation; coefficients give the reacting mole ratio.
- Converting a given mass to moles uses n = m ÷ Mᵣ, with Mᵣ values taken from the periodic table and summed correctly for compounds.
- The mole ratio from the balanced equation is applied to the substance whose mass is required, not assumed to be 1 : 1.
- The final answer is converted from moles back to mass using m = n × Mᵣ and given with a suitable unit such as g or kg.
- Where a reactant is in excess, the limiting reactant determines the amount of product formed.
- Answers should be checked for sensible magnitude and, where appropriate, given to a reasonable number of significant figures.
- 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.
- State that coefficients give the simplest whole-number mole ratio: 1 mol Mg : 2 mol HCl : 1 mol MgCl₂ : 1 mol H₂.
- Explain that subscripts inside a formula, such as the ₂ in H₂, are fixed by the substance and cannot be altered to balance an equation.
- Convert a given mass to moles using moles = mass ÷ relative formula mass, then multiply by the coefficient ratio to find moles of another substance.
- Calculate a reacting mass from the mole ratio, for example 0.10 mol H₂ has a mass of 0.20 g.
- Recognise that the equation alone does not give rate or yield; it gives the stoichiometric amounts that react completely.
- Identify the coefficients 1, 2, 1 and 1 as the mole ratio for Mg, HCl, MgCl₂ and H₂ respectively.
- Convert a stated mass of magnesium to moles using moles = mass ÷ relative atomic mass, then double that value to find moles of HCl.
- Use the same ratio to find the moles of MgCl₂ and H₂ produced, then convert to mass as required.
- Explain that the ratio comes from the balanced equation and applies to particles or moles, not directly to masses of different substances.
- Write or use the balanced symbol equation to identify the stoichiometric ratio between the known and unknown substances.
- Calculate the relative formula mass of both the known and unknown substances using the periodic table.
- Convert the given mass of the known substance into moles using the equation: moles = mass ÷ relative formula mass.
- Apply the molar ratio from the balanced equation to determine the moles of the unknown substance.
- Convert the moles of the unknown substance back into mass using mass = moles × relative formula mass, stating the correct units.
- 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).
Examiner Tips
- 💡Write the balanced equation and label the relevant mole ratio before doing any arithmetic; this makes method marks easier to award.
- 💡Show each step: mass → moles → mole ratio → moles → mass, with units at every stage.
- 💡Check that the relative formula mass you use matches the formula in the equation, but remember that coefficients do not affect the Mᵣ calculation to avoid double-counting.
- 💡If the question gives two reactant masses, identify the limiting reactant before calculating product mass.
- 💡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.
- 💡Underline the coefficients before starting a calculation so you apply the 1:2:1:1 ratio correctly.
- 💡Show the moles of each substance in a clear sequence, then convert only the substance asked for into mass.
- 💡Check that your final answer has the correct unit and a sensible size; a small mass of magnesium should not produce an enormous mass of hydrogen.
- 💡Write the mole ratio underneath the equation before substituting any numbers.
- 💡Keep track of which substance is given and which is asked for, then apply the ratio in the correct direction.
- 💡Always convert mass to moles first, apply the ratio, and then convert back to mass.
- 💡Set out your answer as a clear sequence: equation, relative formula masses, moles of known, molar ratio, moles of unknown, mass of unknown.
- 💡Remember that this specific calculation method using moles and balanced equations is a Higher Tier (HT) only requirement.
- 💡If the question gives a mass in tonnes or kilograms, convert it to grams before using relative formula mass, or be prepared to work in standard form.
- 💡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.
Common Mistakes
- Using an unbalanced equation: this gives the wrong mole ratio. Correction: balance the equation first and use the final coefficients.
- Assuming the mole ratio is always 1 : 1: for example, in 2H₂ + O₂ → 2H₂O the ratio is 2 : 1 : 2. Correction: read the coefficients directly from the balanced equation.
- Multiplying the given mass by Mᵣ instead of dividing: this inverts the conversion. Correction: use n = m ÷ Mᵣ to find moles, then m = n × Mᵣ to return to mass.
- Forgetting to multiply by the coefficient when finding the mass of a product: for example, treating 2MgO as one MgO. Correction: include the coefficient in the mole ratio before converting to mass.
- Reading coefficients as masses rather than moles: for example, treating 2H₂ as 2 g of hydrogen. Correction: coefficients give mole ratios; convert to mass using Mᵣ.
- Ignoring coefficients when scaling: for example, saying 1 mol of O₂ gives 1 mol of H₂O in 2H₂ + O₂ → 2H₂O. Correction: use the coefficient ratio 1 : 2.
- Confusing moles with molecules or atoms: 1 mol of O₂ contains 6.02 × 10²³ molecules of O₂, not 6.02 × 10²³ atoms of oxygen. Correction: state clearly whether the amount refers to molecules, atoms or ions.
- Using an unbalanced equation: this gives an incorrect mole ratio. Correction: balance the equation before interpreting it in moles.
- Reading 2HCl as two hydrogen atoms and two chlorine atoms in one particle; correct this by treating 2HCl as two separate HCl molecules, each with one H and one Cl.
- Changing the subscript in MgCl₂ to MgCl to balance atoms; correct this by placing a coefficient before a formula instead of altering the formula itself.
- Assuming equal masses react in a 1:1 ratio; correct this by converting each mass to moles first, because the mole ratio is 1:2:1:1.
- Treating the coefficient 2 in 2HCl as applying to both H and Cl separately; correct this by remembering it multiplies the whole HCl formula unit.
- Using a 1:1 ratio between magnesium and hydrochloric acid; correct this by reading the coefficients and using 1 mol Mg to 2 mol HCl.
- Adding masses directly from the ratio, such as assuming 1 g Mg gives 2 g HCl; correct this by converting to moles before applying the ratio.
- Forgetting to balance the equation before using the coefficients: correct this by checking that the number of each type of atom is equal on both sides before starting the mole calculation.
- Using the mass ratio directly from the coefficients without converting to moles: correct this by always converting to moles first, because equation coefficients represent mole ratios, not mass ratios.
- Including the balancing coefficient when calculating the relative formula mass: correct this by calculating relative formula mass using only the chemical formula (e.g., for 2H₂O, calculate the mass of H₂O as 18, not 36).
- Rounding too early in the calculation: correct this by keeping full values in your calculator until the final step, then rounding to the appropriate number of significant figures.
- Using the balancing numbers as a mass ratio instead of a mole ratio. Correction: 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. Correction: always 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. Correction: calculate the Mr of each substance separately from its formula.