AQA A-Level Chemistry: Relative Mass, the Mole and the Avogadro Constant — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Relative Mass, the Mole and the Avogadro Constant study guide | Relative Mass, the Mole and the Avogadro Constant revision notes

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Section A

1. Explain why relative atomic mass and relative molecular mass have no units. [2]

2. State the correct term for the relative mass of an ionic compound, and explain why “relative molecular mass” is not used. [2]

Section B

3. Calculate the number of moles in 16.8 g of sodium hydrogencarbonate, NaHCO₃ (Mr = 84). [2]

4. Calculate the number of chloride ions in 200 cm³ of a 0.15 mol dm⁻³ solution of magnesium chloride, MgCl₂, given the Avogadro constant as 6.02 × 10²³ mol⁻¹. [4]

5. A student calculates the mass of calcium carbonate needed to produce a given number of moles, using a mass measured to 3 significant figures and a volume measured to 2 significant figures. State, with a reason, to how many significant figures the final answer should be reported. [2]

6. Ethanoic acid reacts with sodium hydroxide according to the equation CH₃COOH + NaOH → CH₃COONa + H₂O. A student uses 25.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide to exactly neutralise a sample of ethanoic acid. Calculate the number of moles of ethanoic acid present, and the mass of ethanoic acid in the sample (Mr of CH₃COOH = 60). [5]

7. 0.24 g of magnesium reacts completely with excess dilute sulfuric acid according to the equation Mg + H₂SO₄ → MgSO₄ + H₂. Using the GCSE shortcut that 1 mole of any gas occupies 24 000 cm³ at room temperature and pressure (rather than the ideal gas equation) and the Ar of Mg is 24, calculate the volume of hydrogen gas produced. [4]

8. Explain the difference between “amount in moles” and “number of particles”, and describe the single calculation step that converts between them. [3]

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Answers

1. They are defined as ratios against carbon-12 (a mass on a scale where ¹²C = exactly 12) [1], not as an absolute mass measured in grams or any other unit, so the units cancel and no unit is reported [1].

2. Relative formula mass [1]. Ionic compounds such as MgCl₂ do not exist as discrete molecules, so “relative molecular mass” — which strictly applies to covalently bonded molecules — is not the correct term; relative formula mass instead uses the compound’s empirical formula unit [1].

3. Moles = mass ÷ Mr = 16.8 ÷ 84 [1] = 0.2 mol [1].

4. Moles of MgCl₂ = concentration × volume (in dm³) = 0.15 × 0.200 = 0.03 mol [1] [1]. Each formula unit of MgCl₂ contains 2 chloride ions, so moles of Cl⁻ = 0.03 × 2 = 0.06 mol [1]. Number of ions = moles × Avogadro constant = 0.06 × 6.02 × 10²³ = 3.6 × 10²² (2 s.f., matching the 0.15 mol dm⁻³ data) [1].

5. 2 significant figures [1], because the final answer must be reported only to the limits of the least accurate (least precise) measurement used, and the volume’s 2 significant figures is less precise than the mass’s 3 significant figures [1].

6. Moles of NaOH = concentration × volume = 0.100 × 0.0250 = 0.00250 mol [1] [1]. The equation shows a 1:1 mole ratio between NaOH and CH₃COOH, so moles of ethanoic acid = 0.00250 mol [1]. Mass = moles × Mr = 0.00250 × 60 = 0.15 g [1] [1].

7. Moles of Mg = mass ÷ Ar = 0.24 ÷ 24 = 0.01 mol [1]. The equation shows a 1:1 mole ratio between Mg and H₂, so moles of H₂ produced = 0.01 mol [1]. Volume = moles × molar volume = 0.01 × 24 000 [1] = 240 cm³ [1].

8. Amount in moles measures the quantity of substance using the mole as the counting unit, while number of particles is the actual count of individual atoms, molecules or ions [1] [1]. The two are related by the Avogadro constant: number of particles = moles × Avogadro constant (or moles = number of particles ÷ Avogadro constant) [1].

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Where marks are usually lost

• Forgetting to convert cm³ to dm³ before using a mol dm⁻³ concentration.
• Missing the ionic-formula multiplier (e.g. two chloride ions per MgCl₂ formula unit) when converting moles of compound to moles of ion.
• Using “relative molecular mass” for an ionic compound instead of relative formula mass.
• Rounding a final answer to more significant figures than the least precise given measurement allows.
• Forgetting to apply the mole ratio from a balanced equation when moving from one substance to another.

Approaching amount of substance calculations

Treat every calculation as a short chain of separate, labelled steps rather than one combined formula: convert any volume to the correct unit first, calculate moles of the substance given, apply the balanced equation’s mole ratio if the question moves from one substance to another, and only then convert to the quantity actually asked for (mass, number of particles, or volume of gas). Writing out each step explicitly, even for straightforward questions, both avoids the most common errors above and secures method marks under AQA’s mark scheme even when a final numerical slip occurs.

Before starting a multi-step question, identify which of the three core relationships — mass and Mr, concentration and volume, or moles and the Avogadro constant — the given data and the final target both belong to, since most errors come from applying the wrong pairing rather than an arithmetic slip. When a question involves a balanced equation, write the mole ratio out explicitly as a fraction (for example, 1 mol Mg : 1 mol H₂) rather than assuming a 1:1 relationship by default, since many equations in later topics use different ratios and this habit prevents a costly assumption error. Finally, check the final answer’s significant figures against the least precise piece of data supplied in the question as a separate, deliberate last step, since AQA awards this as its own mark independent of whether the calculation method itself was correct.