All calculations
Missing mass by conservation
Quantitative chemistry
AQA 8462
ORIGINAL
total mass of reactants = total mass of products
In a closed system the totals on both sides must match; identify the missing term before subtracting.
Verified against AQA 8462 (2026 spec)
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
2 marks
In a closed container, 4.8 g of magnesium reacts to form 8.0 g of magnesium oxide. Calculate the mass of oxygen that reacted.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Adding when the missing reactant mass must be found by subtraction.
- Applying conservation to an open system where gas can enter or escape.
Exam tip: Draw a one-line mass balance and label whether the apparatus is open or closed.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.