Relative formula mass — AQA GCSE Chemistry

Relative formula mass (represented as Mr) is the sum of the relative atomic masses (Ar) of all the atoms present in a given chemical formula. It applies to both simple molecular substances like carbon dioxide and giant ionic structures like sodium chloride.

Mastering Mr calculations is the cornerstone of GCSE quantitative chemistry. It serves as the primary gateway calculation required for determining moles, theoretical yields, percentage composition by mass, and titration stoichiometry.

• →Relative atomic mass (Ar) is found on the Periodic Table as the top/larger number for each element (mass number).
• →Subscripts inside a chemical formula indicate the exact number of atoms of that preceding element; brackets multiply everything inside them.
• →Relative formula mass is a ratio compared to carbon-12 and therefore does not have units.
• →Balancing numbers (stoichiometric coefficients) placed in front of chemical formulas are ignored when calculating standard Mr.

• Relative formula mass is the sum of relative atomic masses of all atoms in the formula.
• Each species' Mᵣ must be multiplied by its coefficient in the balanced equation.
• The total Mᵣ of reactants equals the total Mᵣ of products when the equation is balanced.
• Conservation of mass follows from equal numbers of each atom on both sides.
• Worked check: 2H₂ + O₂ → 2H₂O gives 36 = 36 using Mᵣ values H₂ = 2, O₂ = 32, H₂O = 18.
• This principle underpins reacting mass and percentage yield calculations.
• Calculate Mᵣ by summing the relative atomic masses of all atoms in the formula.
• Find the total mass of the chosen element using its relative atomic mass and atom count.
• Divide the element's total mass by Mᵣ and multiply by 100 to obtain percentage by mass.
• Worked example: in CO₂, carbon is 12 ÷ 44 × 100 = 27.3% and oxygen is 32 ÷ 44 × 100 = 72.7%.
• The percentages of all elements in a compound should sum to 100%.
• Give answers to an appropriate number of significant figures, typically three.

• 💡Write the Mᵣ of each species clearly beside its formula before multiplying by coefficients.
• 💡Show the multiplication step, for example 2 × 18 = 36, so the examiner can follow your reasoning.
• 💡Check that the equation is balanced before using it for mass calculations.
• 💡Use a periodic table with relative atomic masses and round consistently as instructed.
• 💡Write down Mᵣ first and label it clearly before starting the percentage calculation.
• 💡Show the numerator as the element's total mass and the denominator as Mᵣ, then multiply by 100.
• 💡Check that the percentages for all elements add up to 100% as a final verification.
• 💡State the element and the compound in your answer, for example 'percentage by mass of carbon in CO₂'.
• 💡Always show full working out showing the sum of (Ar x count) so you can still earn method marks if you make an arithmetic slip on your calculator.
• 💡Double check values from the provided AQA Periodic Table — do not guess or memorise Ar values, as small differences (e.g., Cu = 63.5, Cl = 35.5) will penalise you.

• Forgetting to multiply Mᵣ by the coefficient, so comparing 2 + 32 with 18; the correction is to include coefficients, giving 36 = 36.
• Adding relative atomic masses incorrectly, such as using the wrong value for an element; the correction is to use the periodic table values accurately and sum every atom.
• Assuming an unbalanced equation still conserves mass; the correction is that coefficients must first balance the equation before totals can be compared.
• Dividing by the element's mass instead of Mᵣ; the correction is to divide the element's total mass by the compound's Mᵣ.
• Forgetting to multiply the relative atomic mass by the number of atoms of that element; the correction is to include the atom count, for example 2 × 16 for oxygen in CO₂.
• Rounding intermediate values too early, causing a final answer that is slightly wrong; the correction is to keep full values during the calculation and round only at the end.
• Using the atomic number (bottom number) instead of the relative atomic mass (top number) on the Periodic Table.
• Adding coefficients into the Mr calculation (e.g., treating 3CO2 as having an Mr of 3 x 44 = 132 instead of 44).
• Multiplying parts of a hydrated salt formula rather than adding (e.g., treating the dot in CuSO4.5H2O as a multiplication sign instead of adding 5 waters).

1. 1Day 1: Practice identifying the mass number (Ar) versus atomic number on the AQA Periodic Table for common elements.
2. 2Day 2: Work through 10 basic compounds containing no brackets (e.g., CO2, H2SO4, NaCl).
3. 3Day 3: Practice 10 complex compounds containing brackets and hydrated salts (e.g., Mg(NO3)2, Fe2(SO4)3, Na2CO3.10H2O).
4. 4Day 4: Attempt exam questions combining Mr calculations with percentage by mass and mole calculations.

• 📋Direct 1-2 mark retrieval questions asking to calculate the Mr of a named or drawn compound.
• 📋Multi-step quantitative questions where calculating Mr is an unprompted, essential first step to find moles or reacting mass.
• 📋Percentage by mass questions where students must divide the total Ar of one element by the whole compound Mr and multiply by 100.