Magnesium burns in oxygen: 2Mg + O2 → 2MgO. Calculate the mass of magnesium oxide formed when 6.0 g of magnesium burns completely. (Ar of Mg = 24; Mr of MgO = 40)
[4 marks]
Total for this question: 4
All calculations
Reacting masses
Quantitative chemistry
AQA 8462
ORIGINAL
Higher tier
mass → moles (n = m/Mr) → ratio from equation → moles → mass (m = n x Mr)
Mass doesn't go straight to mass — you cross the 'moles bridge' using the balanced equation.
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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
- Going straight from mass to mass without converting to moles.
- Forgetting the mole ratio from the balanced equation.
- Using the Ar of the element when the Mr of the compound is needed (or vice versa).
Exam tip: Every reacting-mass question is the same four steps: mass → moles → ratio → moles → mass. Write the balanced equation above your working so the ratio is in front of you.
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