Relative Formula Mass and Conservation of Mass — AQA GCSE Chemistry
Quantitative chemistry begins with calculating masses and understanding that mass is conserved in reactions.
Conservation of mass
In a chemical reaction, no atoms are created or destroyed, so the total mass of the products equals the total mass of the reactants. This is why symbol equations must be balanced.
Mass can sometimes appear to change:
- A reaction that gives off a gas appears to lose mass (the gas escapes).
- A reaction where a substance gains a gas from the air appears to gain mass. In a sealed container, the mass stays the same.
Relative formula mass (Mr)
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms shown in its formula.
Worked examples
- Water, H₂O: Mr = (2 × 1) + 16 = 18.
- Carbon dioxide, CO₂: Mr = 12 + (2 × 16) = 44.
- Calcium hydroxide, Ca(OH)₂: Mr = 40 + 2×(16 + 1) = 74.
In a balanced equation, the sum of the Mr of the reactants = sum of the Mr of the products — a check on conservation of mass.
Exam tips
- State conservation of mass: total mass of products = total mass of reactants.
- Explain "apparent" mass changes by a gas being lost or gained.
- Calculate Mr by adding up all the Ar values (watch brackets, e.g. Ca(OH)₂).
- Balanced equations must obey conservation of mass.