AQA A-Level Chemistry: Relative Mass, the Mole and the Avogadro Constant (7405)

This guide covers 3.1.2.1 Relative atomic mass and relative molecular mass and 3.1.2.2 The mole and the Avogadro constant, from AQA A-level Chemistry (7405), for teaching from September 2015. These two sub-topics open 3.1.2 Amount of Substance, AS-shared content within 3.1 Physical Chemistry.

Scope of this guide

3.1.2 Amount of Substance has five sub-topics: 3.1.2.1 and 3.1.2.2 (covered here), plus 3.1.2.3 The Ideal Gas Equation, 3.1.2.4 Empirical and Molecular Formula, and 3.1.2.5 Balanced Equations and Associated Calculations (percentage yield, atom economy and titration calculations). This resource focuses on the foundational mass and mole concepts that the later sub-topics build on directly; the ideal gas equation, empirical/molecular formula and balanced-equation calculations are left for separate resources.

Syllabus coverage

AQA A-LEVEL CHEMISTRY (7405) — 3.1.2.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS

Relative atomic mass and relative molecular mass are defined in terms of carbon-12 (¹²C). The term relative formula mass is used for ionic compounds instead of relative molecular mass. Students should be able to define relative atomic mass (Ar) and relative molecular mass (Mr).

3.1.2.2 THE MOLE AND THE AVOGADRO CONSTANT

The Avogadro constant is the number of particles in a mole. The mole applies to electrons, atoms, molecules, ions, formulas and equations. The concentration of a substance in solution is measured in mol dm⁻³. Students should be able to carry out calculations using the Avogadro constant, using mass of substance/Mr/amount in moles, and using concentration/volume/amount of substance in a solution. Students are not expected to recall the value of the Avogadro constant, and are expected to report calculated results only to the limits of the least accurate measurement used.

How to approach it

Treat Ar and Mr as ratios against carbon-12, not as raw masses — this is why they have no units. The specification explicitly names relative formula mass as the correct term for ionic compounds (such as sodium chloride) rather than relative molecular mass, which strictly applies to covalently bonded molecules; using the wrong term for an ionic compound is a small but real accuracy issue examiners look for.

For 3.1.2.2, the three calculation types the specification names — using the Avogadro constant directly, using mass/Mr/moles, and using concentration/volume/moles — should each be practised as their own rearrangeable triangle or formula, since exam questions typically give two quantities and ask for the third:

moles = mass ÷ Mr
moles = concentration × volume (in dm³)
number of particles = moles × Avogadro constant

Because the Avogadro constant itself is not required to be memorised, questions that need it will provide its value — so the skill being tested is applying it correctly in a calculation, not recalling it.

Worked example: concentration to number of particles

A question gives 250 cm³ of a sodium chloride solution with a concentration of 0.20 mol dm⁻³ and asks for the number of ions present, given the Avogadro constant as 6.02 × 10²³ mol⁻¹.

Step 1: convert volume to dm3
        250 cm3 = 0.250 dm3

Step 2: calculate moles of NaCl
        moles = concentration x volume = 0.20 x 0.250 = 0.050 mol

Step 3: convert moles to number of formula units
        number of formula units = moles x Avogadro constant
                            = 0.050 x 6.02x10^23 = 3.0x10^22 (2 s.f., matching the 0.20 mol dm-3 data)

Step 4: apply the ionic multiplier
        NaCl gives 2 ions per formula unit (Na+ and Cl-), so
        number of ions = 2 x 3.0x10^22 = 6.0x10^22

Every step should be shown explicitly, since AQA mark schemes award method marks for correct intermediate steps even if the final answer is wrong due to a small arithmetic slip. Naming the solute matters: the number of ions depends on how many ions each formula unit releases, so the same moles of a solute like magnesium chloride (three ions per formula unit) would give a different final answer.

Key terms to define precisely

Relative atomic mass (Ar) — the weighted mean mass of an atom of an element, measured on a scale where an atom of carbon-12 has a mass of exactly 12, taking into account the relative abundance of the element’s isotopes. Relative molecular mass (Mr) — the weighted mean mass of a molecule, measured on the same carbon-12 scale, calculated as the sum of the relative atomic masses of all atoms in the molecular formula. Relative formula mass — the equivalent quantity for an ionic compound, calculated the same way as Mr but using the compound’s empirical formula unit rather than a true molecule, since ionic compounds do not exist as discrete molecules. Mole — the SI unit of amount of substance, defined such that one mole of any substance contains the same number of specified elementary entities (atoms, molecules, ions or electrons) as the Avogadro constant. Avogadro constant — the number of particles in one mole of any substance, approximately 6.02 x 10²³ per mole. Keeping “amount in moles” and “number of particles” as two clearly separate quantities related by the Avogadro constant, rather than treating them as interchangeable, avoids one of the most common calculation errors in this sub-topic.

Common mistakes

Forgetting to convert cm³ to dm³ before using a concentration in mol dm⁻³. Using “relative molecular mass” for an ionic compound instead of the specification’s preferred term, relative formula mass. Reporting a calculated answer to more significant figures than the least accurate piece of data supplied in the question allows. Confusing the number of moles with the number of particles when the Avogadro constant step is skipped.

Quick revision checklist

• Be able to define Ar and Mr precisely, in terms of carbon-12.
• Know when to use “relative formula mass” instead of “relative molecular mass.”
• Practise all three mole-calculation triangles: mass/Mr, moles x Avogadro constant, and concentration x volume.
• Always convert cm³ to dm³ before using a mol dm⁻³ concentration.
• Round final answers to the appropriate number of significant figures based on the least precise given value.

Related resources

Relative Mass, the Mole and the Avogadro Constant revision notes | Relative Mass, the Mole and the Avogadro Constant practice questions

Official syllabus

AQA A-level Chemistry (7405) specification, for teaching from September 2015 — aqa.org.uk/7405.