GCSE Chemistry · AQA 8462 · Topic 4.3.1
Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations
Key points
- The law of conservation of mass: no atoms are created or destroyed, so the mass of products equals the mass of reactants. This is why equations must be balanced.
- Relative formula mass () is found by adding the relative atomic masses () of all the atoms in the formula, e.g. of . In a balanced equation, the sum of of reactants equals that of products.
- If a reactant is a gas from the air (e.g. oxygen when a metal burns), the mass of the solid can increase. If a product is a gas that escapes (e.g. from a carbonate), the mass in an open container decreases.
- Every measurement has some uncertainty. The range of repeat results about the mean gives an estimate: uncertainty ≈ half the range.
- Always give answers with sensible significant figures and units.
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This sub-topic sets out the rules every chemical calculation relies on: mass is conserved, formula masses come from relative atomic masses, and every measurement has some uncertainty.
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