Stoichiometry & The Mole Concept
Chemistry stoichiometry hub: formulas, balanced equations, mole calculations, concentration, limiting reagent, and yield/purity. Practise the topic with its check.
Before you begin
Chemical calculations link equations to real amounts of reactants and products. The lessons start with symbols and equations, build the mole concept, then apply it to formulae, concentrations, limiting reactants, and yield and purity; each calculation builds on the conversion before it.
Be comfortable with: elements, compounds and mixtures, atoms and the unit conversions in measurement and experimental techniques.
Learning goals
- state the symbols of the elements and formulae of the compounds mentioned in the syllabus
- deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa
- deduce the formulae of ionic compounds from the charges on the ions present and vice versa
- interpret chemical equations with state symbols
- construct chemical equations, with state symbols, including ionic equations.
- define relative atomic mass, Ar
- define relative molecular mass, Mr, and calculate relative molecular mass (and relative formula mass) as the sum of relative atomic masses
- define the term mole in terms of the Avogadro constant
- calculate the percentage mass of an element in a compound when given appropriate information
- calculate empirical and molecular formulae from relevant data
- calculate stoichiometric reacting masses and volumes of gases (one mole of gas occupies 24 dm3 at room temperature and pressure); calculations involving the idea of limiting reactants may be set (knowledge of the gas laws and the calculations of gaseous volumes at different temperatures and pressures are not required)
- apply the concept of solution concentration (in mol/dm3 or g/dm3) to process the results of volumetric experiments (e.g. titration) and to solve simple problems (appropriate guidance will be provided where unfamiliar reactions are involved)
- calculate % yield and % purity.
Lessons
Work through them in order.
Symbols and equations
Amounts and formulae
- Relative atomic, molecular and formula massCalculate relative atomic and relative molecular masses.
- Moles, molar mass and particle countsConvert between mass and moles using molar mass.
- Percentage composition by massCalculate the percentage by mass of each element in a compound.
- Finding empirical and molecular formulaeWork out empirical and molecular formulae from composition data.
Reaction calculations
- Gas volumes and reacting amountsConvert gas volumes to moles and use equations to find reacting amounts.
- Solution concentration and titration calculationsUse mass and molar concentration, conserve solute amount during dilution, and calculate an unknown concentration from a titration and its balanced equation.
- Limiting reactants and excess left overIdentify the limiting reactant and calculate the most product it allows.
- Percentage yield: actual versus theoreticalCompare actual and theoretical yield, and distinguish yield from purity.
- Percentage purity: finding the pure partFind the pure substance in an impure sample from gas or titration data, calculate percentage purity, and combine purity with percentage yield.
Practise and check
Recommended nextChemical Calculations topic check
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Topic reference
Quick Reference
| Item | Quick rule / reminder |
|---|---|
| Moles from mass | n = m/M (use M in g mol⁻¹) |
| Moles ↔ particles | N = nN_A and n = N/N_A (use the rounded N_A ≈ 6.02 × 10²³ mol⁻¹ unless given another value) |
| Moles from gas volume (RTP) | n = V/Vₘ (use Vₘ = 24 dm³ mol⁻¹ with V in dm³) |
| Molar concentration | c = n/V (use V in dm³) |
| Rearranging concentration | n = cV and V = n/c |
| Volume conversions | 1 dm³ = 1000 cm³ so 24 dm³ = 24000 cm³ |
| Percentage yield | % yield = actual/theoretical × 100% |
| Percentage purity | % purity = (mass of pure substance)/(mass of sample) × 100% |
Core knowledge to remember
- Mole: unit of amount of substance. One mole contains exactly 6.02214076 × 10²³ specified entities; use the supplied or appropriate rounded value for calculations.
- Relative atomic mass (Aᵣ): weighted average mass of an atom compared to 1/12 of ¹²C.
- Relative molecular mass (Mᵣ): sum of Aᵣ values in one molecule; use relative formula mass for an ionic formula unit.
- Molar mass: mass of 1 mole of a substance (g mol⁻¹).
- Empirical formula: simplest whole-number ratio of atoms present.
- Molar concentration (c): moles of solute per dm³ of solution (mol dm⁻³).
- Limiting reactant: reactant that is used up first, limiting product formed.
- Percentage yield: actual yield as a percentage of theoretical yield.
Common mistakes
- Mole ratio step: always use a balanced equation first.
- Units: convert volumes to dm³ before using c = n/V.
- Relative-mass labels: Aᵣ is an element’s average relative atomic mass, Mᵣ is relative molecular mass, and relative formula mass applies to a formula ratio for a substance without separate molecules. All are unitless.
- Limiting reactant: product amount depends on the reactant that runs out first.
- Yield vs purity: yield compares actual vs theoretical; purity compares pure vs sample mass.
- Rounding too early: keep guard digits through intermediate steps and round only the final answer to the precision justified by the data.
- Mass ratio mistake: use moles, not masses, to compare reactants (unless molar masses are the same).
- Gas volume units: 24 dm³ at r.t.p., not 24 cm³.