AQA gas-volume questions stay with reacting gas-volume ratios rather than using molar gas volume. AQA still uses the ideal gas equation when that route is required.
AQA · OCR A · Amount of substance
A-level moles questions and practice
Choose the bit that is actually costing you marks: basic amount calculations, mole ratios, reacting masses, solutions, gas volumes, limiting reagents, formulae, hydrated salts, yield or harder mixed questions.
Choose a skill
Amount of substance is a chain of small decisions
Pick a skill and you’ll get generated questions with worked feedback. Board controls change only where AQA and OCR A genuinely differ.
Mass, Mr & moles
Move confidently between mass, molar mass, amount and particle number.
n = m/Mrmass ↔ molesAvogadro constant
Practise
Mole ratios
Use balanced equations to convert one reacting amount into another.
balanced equationscoefficientsstoichiometric ratio
Practise
Reacting masses
Mass → moles → equation ratio → mass, including decomposition and multi-stage variants.
mass → molesmole ratiomoles → mass
Practise
Solutions & concentration
Use n = cV, prepare solutions and handle dilution without hiding the units.
n = cVcm³ ↔ dm³dilutionmass concentration
Practise
Gas volumes
Practise gas-volume ratios directly; OCR A mode also uses molar gas volume from the data sheet.
gas-volume ratiogas stoichiometryOCR A molar gas volume
Practise
Limiting reagents
Compare reacting amounts properly, calculate product and find reagent left in excess.
limiting reagentmoles ÷ coefficientexcess remaining
Practise
Empirical & molecular formula
Turn masses or percentages into simplest ratios, then use Mr to obtain molecular formulae.
empirical formulawhole-number ratiomolecular formula
Practise
Hydrated salts
Find x in salt·xH₂O from mass loss or percentage composition.
mass lossmole ratiowater of crystallisation
Practise
Yield & atom economy
Calculate theoretical yield, percentage yield and atom economy from balanced equations.
theoretical yieldpercentage yieldatom economy
Practise
Hard mixed questions
Less signposted multi-stage amount-of-substance problems combining several skills.
route selectionstoichiometryunitsmulti-stage reasoning
Practise
Ideal gas equationpV = nRT, unit conversions, Mr, decomposition and unfamiliar gas problems.
pV = nRTPam³K
Open established tool
A useful mental route
Most moles questions are conversions linked by an equation
The difficult part is often identifying which conversion is needed next. A mass becomes moles through Mr; a solution volume becomes moles through concentration; gas volumes can sometimes be handled directly from reacting volume ratios; OCR A also uses molar gas volume, and both boards use the ideal gas equation where appropriate. Only then does the balanced equation tell you how to move from one substance to another.
The practice tools keep those stages explicit so a wrong answer can be tied to a real underlying skill rather than just labelled “moles”.
Convert inmass · solution · gas · particles
→
Amountmoles
→
Reactbalanced equation ratio
→
Convert outmass · concentration · gas · formula
Board differences worth knowing
Same chemistry, slightly different exam furniture
The OCR A data sheet gives a molar gas volume of 24.0 dm³ mol⁻¹. If a question supplies a different molar gas volume, use the value in that question.
OCR A uses R = 8.314 J K⁻¹ mol⁻¹ from its data sheet. AQA questions supply R = 8.31 J K⁻¹ mol⁻¹ in the question.
Mass, moles, equation ratios and most stoichiometry are the same calculation. The board selector is only used when wording, supplied data or conventions genuinely differ.