AQA · Chemistry · 8462
GCSE Chemistry: Relative formula mass
Quantitative chemistry · AQA GCSE Chemistry (8462), specification point 4.3.1.2. Cards are generated from the exam board's own specification for "Relative formula mass", and every card links back to the page it came from. Specification © AQA — used with attribution.
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What does the symbol Mr stand for in chemistry?
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Relative formula mass.
How is the relative formula mass (Mr) of a compound calculated?
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By adding up the relative atomic masses (Ar) of all the atoms in the numbers shown in the formula.
The relative formula mass of a compound is the ? of the relative atomic masses of the atoms in the numbers shown in the formula.
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sum
Calculate the Mr of water, H₂O (Ar: H = 1, O = 16).
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18 — (2 × 1) + 16 = 18.
Calculate the Mr of calcium carbonate, CaCO₃ (Ar: Ca = 40, C = 12, O = 16).
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100 — 40 + 12 + (3 × 16) = 100.
In a balanced chemical equation, how does the total Mr of the reactants compare with the total Mr of the products?
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They are equal — the sum of the relative formula masses of the reactants (in the quantities shown) equals the sum for the products, because mass is conserved.
Why is the total Mr of reactants equal to the total Mr of products in a balanced equation?
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Because atoms are neither created nor destroyed in a chemical reaction — the same atoms are simply rearranged, so total mass is conserved.
What is the formula for the percentage by mass of an element in a compound?
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% by mass = (Ar of element × number of atoms of that element in the formula) ÷ Mr of compound × 100.
What two quantities are needed to calculate the percentage by mass of an element in a compound?
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The relative formula mass (Mr) of the compound and the relative atomic mass (Ar) of the element.
Calculate the percentage by mass of oxygen in water, H₂O (Ar: H = 1, O = 16).
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88.9% — (16 ÷ 18) × 100 ≈ 88.9%.
Calculate the percentage by mass of carbon in CO₂ (Ar: C = 12, O = 16).
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27.3% — Mr = 44; (12 ÷ 44) × 100 ≈ 27.3%.
When calculating the Mr of Mg(OH)₂, how many oxygen atoms and hydrogen atoms must be counted?
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2 oxygen and 2 hydrogen — the subscript outside the bracket multiplies everything inside it, so Mr = 24 + 2 × (16 + 1) = 58.
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