Stoichiometry

Practise H2 Chemistry mole and formula calculations, limiting reagents, concentrations, titrations and gas quantities through ordered lessons.

GCE A-Level H2 Chemistry 2027
6 lessons

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Before you begin

Every calculation in this topic follows one method: convert what you know to moles, use the mole ratio from the balanced equation, then convert to what the question asks for. The lessons start with the mole and formulae, then apply the method to reacting masses, solutions, titrations and gases.

Be comfortable with: writing and balancing equations, the mole at O-Level, rearranging equations, and significant figures.

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.

Practise and check

Or choose

Topic reference

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 = nNA	NA = 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	8.31 J mol⁻¹K⁻¹ with Pa and m³; 8.31 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

Core knowledge to remember

A mole contains 6.02 × 10²³ particles; N = nNA. Always state the particle: atoms, molecules, ions or formula units.
n = m/M, with the molar mass M in g mol⁻¹; relative masses have no unit.
The empirical formula is the simplest whole-number ratio of atoms; the molecular formula is a whole-number multiple of it.
The limiting reagent is used up first and decides the amount of product.
n = cV with V in dm³; c₁V₁ = c₂V₂ for a dilution only.
pV = nRT with T in kelvin; gases at the same temperature and pressure react in the same ratio by volume as by moles.

Common mistakes

Using a volume in cm³ in n = cV or pV = nRT without converting.
Giving Mᵣ the unit of molar mass.
Skipping the balanced equation, and so using the wrong mole ratio.
Using c₁V₁ = c₂V₂ when the solutions react.
Using the Celsius temperature in pV = nRT.
Including the rough titre in the mean titre.
Rounding in the middle of a calculation.