Relative Masses, Moles and Stoichiometry
Relative atomic and formula mass, isotope abundance, the mole, Avogadro constant, reacting ratios and limiting reactants.
How to study for GCSE Chemistry
Move between particle models, equations, calculations, observations and data; practise the specific required activities and paper structure named by your exam board.
Core concepts
Concept 1
Relative atomic mass is a weighted mean of isotope masses, while relative formula mass is the sum for the formula represented.
Exam cue: Write the chemical equation and convert quantities to moles before applying the stoichiometric ratio.
Concept 2
Amount in moles equals mass divided by molar mass, connecting measured mass to number of particles through the Avogadro constant.
Exam cue: Track units and convert back to the quantity requested only after the mole comparison.
Concept 3
Balanced-equation coefficients give mole ratios; the limiting reactant is exhausted first and fixes the maximum product amount.
Exam cue: For two reactants, compare available moles with the required ratio to identify the limiting one.
Risk pitfalls and guardrails
Averaging isotope masses without weighting their abundances.
Guardrail: Do not rely on a memorised keyword alone: preserve charges, ratios, units and conditions, and keep single Chemistry separate from Combined Science.
Using mass ratios directly where the equation provides mole ratios.
Guardrail: Do not rely on a memorised keyword alone: preserve charges, ratios, units and conditions, and keep single Chemistry separate from Combined Science.
Assuming the reactant with the smaller mass is the limiting reactant.
Guardrail: Do not rely on a memorised keyword alone: preserve charges, ratios, units and conditions, and keep single Chemistry separate from Combined Science.
Memory anchors
Relative atomic mass
The abundance-weighted mean mass of an element's atoms relative to carbon-12.
Relative formula mass
The sum of relative atomic masses in the stated formula.
Mole
The amount containing the Avogadro number of specified particles.
Moles from mass
Amount in mol = mass in g ÷ molar mass in g/mol.
Limiting reactant
The reactant used up first, which limits the amount of product.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What does one mole contain?
How many moles are in 18 g water, Mr 18?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
What is Pass Harbor?
Completely free exam prep for 247 UK exams.
- Practice questions
- Flashcards
- Study guides
- Mock exams
- No registration
- No paywall
- Start instantly
“No more expensive exam prep. Quality study tools should be accessible to everyone.”