Reacting masses
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.
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.
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)
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.
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.