Stoichiometry & The Mole Concept
Chemistry stoichiometry hub: formulas, balanced equations, mole calculations, concentration, limiting reagent, and yield/purity. Includes quizzes.
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.
Stoichiometry is the quantitative “math” of chemistry: it links chemical equations to real amounts of reactants and products. This topic builds the mole concept, then applies it to formulas, concentrations, limiting reactants, and yield/purity calculations.
Jump to: Prerequisites · Quick Reference · Core knowledge · Sub-topics · Quiz
Prerequisites
Revise Elements, Compounds & Mixtures, Atoms, and the unit conversions in Measurements & Experimental Techniques.
New to the topic? Begin with Chemical Symbols, Chemical Equations, Ionic Equations, Atomic Mass, then Mole & Molar Mass. Continue through the Sub-topics in order because each calculation builds on the previous conversion.
For revision, use the Quick Reference, Core knowledge, and Common mistakes. For practical work, use the K324 / 6092 Chemistry Practical Hub.
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 N_A = 6.02 × 10²³ mol⁻¹) |
| Moles from gas volume (r.t.p.) | n = V/(24 dm³) (use 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: amount of substance containing the Avogadro constant, 6.02 × 10²³, specified entities.
- 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.
Sub-topics
Writing Chemical Equations
How To Write Chemical Symbols & Formulas
Write formulas fast using ion charges.
How To Write A Chemical Equation
Balance equations without changing subscripts.
How To Write Ionic Equations
Cancel spectators and balance charge.
The Mole Concept
Atomic Mass & Molecular Mass
Relative masses (Aᵣ, Mᵣ).
Mole & Molar Mass
The mole unit and calculations.
Chemical Calculations
Percentage Composition
Mass percent of elements.
Empirical & Molecular Formula
Determining formulas from data.
Molar Volume & Concentration
Gas volumes and solution concentration.
Limiting Reactant
Identifying the reactant that limits yield.
Percentage Yield & Percentage Purity
Actual vs theoretical; pure vs sample.
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 for one atom, Mᵣ for one molecule, and relative formula mass for an ionic formula unit.
- 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.