Moles from mass
n = m / Mr (moles = mass / relative formula mass)
n = m / Mr underpins nearly every calculation — get this automatic and the rest follow.
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.
Calculate the number of moles in 8.0 g of sodium hydroxide, NaOH. (Mr of NaOH = 40)
Do the calculation on paper first — then mark it.
Calculate the mass of 0.25 mol of calcium carbonate, CaCO3. (Mr of CaCO3 = 100)
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using an Ar (atomic) instead of the Mr (formula) mass for a compound.
- Leaving the mass in a non-gram unit — convert to grams first.
- Rounding the moles too early and carrying the error into the next step.
Exam tip: Write the triangle n = m / Mr and cover the quantity you want. Keep 3+ significant figures in your working and only round the final answer.
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.