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.
[2 marks]
Total for this question: 2
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.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
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
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. Free intro call, then a free first lesson.
Stuck on conservation missing mass?
Everything here stays free. If conservation missing mass is where the marks keep going, I teach it one to one from the mark schemes. Free intro call, then a free first lesson.