Stoichiometry
Moles, formulae, reacting quantities, solutions and gas calculations.
Before you begin
Moles, formulae, reacting quantities, solutions and gas calculations.
Learning goals
- Mole and Avogadro Constant
- Empirical and Molecular Formula
- Reacting Masses and Limiting Reagent
- Solution Concentration and Dilution
- Titration Calculations
- Gas Calculations (pV = nRT)
Syllabus statements covered
- define the terms relative atomic, isotopic, molecular and formula mass
- define the term mole in terms of the Avogadro constant
- calculate the relative atomic mass of an element given the relative abundances of its isotopes
- define the terms empirical and molecular formula
- calculate empirical and molecular formulae using combustion data or composition by mass
- write and/or construct balanced equations
- perform calculations, including use of the mole concept, involving: — reacting masses (from formulae and equations)
- deduce stoichiometric relationships from calculations such as those in (g)
- perform calculations, including use of the mole concept, involving: — volumes and concentrations of solutions [when performing calculations, candidates’ answers should reflect the number of significant figures given or asked for in the question]
- perform calculations, including use of the mole concept, involving: — volumes of gases (e.g. in the burning of hydrocarbons)
Lessons
Work through them in order.
- Mole and Avogadro ConstantConvert between particles, moles and mass using the Avogadro constant and molar mass.
- Empirical and Molecular FormulaCalculate empirical and molecular formulae from composition data.
- Reacting Masses and Limiting ReagentFind the limiting reagent and calculate the product yield and the reactant left over.
- Solution Concentration and DilutionUse n = cV and the dilution rule, converting volume units correctly.
- Titration CalculationsCalculate amounts and concentrations from titration data using the equation's mole ratio.
- Gas Calculations (pV = nRT)Combine pV = nRT with reacting ratios to calculate gas quantities.
Practise and check
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Topic reference
Stoichiometry turns calculation questions into one repeatable method: convert to moles, use the equation ratio, then convert back. The lessons go from the mole and formulae to reacting masses, solutions, titrations and gases.
Be comfortable with:
- How to Write a Chemical Equation: balancing equations.
- Mole & Molar Mass: O Level mole basics.
- Rearranging algebra and using significant figures.
Quick Reference
| What you need | Use | Unit check |
|---|---|---|
| universal workflow | balance → convert to moles → ratio → convert to asked quantity | units and sig figs stay consistent |
| particles ↔ moles | N = nN_A | N_A = 6.02 × 10²³ mol⁻¹ (Data Booklet) |
| mass ↔ moles | n = m/M | M in g mol⁻¹ |
| solution moles | n = cV | V in dm³ |
| dilution | c₁V₁ = c₂V₂ | same solute only (no reaction) |
| gas moles | pV = nRT | T in K; match units to R |
| common R values | 8.314 J mol⁻¹K⁻¹ with Pa and m³; 8.314 kPa dm³ mol⁻¹K⁻¹ with kPa and dm³ | pick R to match your units |
| titration data | mean of concordant titres; ignore rough; record burette to 2 d.p. | check question’s concordance tolerance |
What You Must Memorise
- N = nN_A and n = N/N_A (state the entity: atoms/molecules/ions/formula units).
- n = m/M (use M in g mol⁻¹; don’t treat Mᵣ as a unit).
- n = cV with V in dm³ (convert: 1000 cm³ = 1 dm³).
- c₁V₁ = c₂V₂ is for dilution only (no reaction; same solute).
- pV = nRT with T in K and R matching your p and V units.
- Sig figs: keep 3–4 s.f. through working; round at the end unless instructed.
Common Exam Traps
- Mixing cm³ and dm³ (and not converting).
- Using Mᵣ (no units) instead of molar mass, M (with units).
- Skipping the balanced equation, then using the wrong mole ratio.
- Rounding too early (losing marks in multi-step calculations).
- Using c₁V₁ = c₂V₂ when a reaction happens (it’s for dilution only).
- Using pV = nRT with T in °C (must be K).
- Mixing “moles of gas” vs “volume of gas”, or mixing dm³ with cm³ inside pV = nRT.
- Using average titre incorrectly (not discarding rough/outliers when instructed).
- Quoting answers to inconsistent significant figures.