Unit Overview

Stoichiometry

CIE IGCSE Chemistry· 5 min read 📊 12-15% of total exam, across MCQ, structured and extended response sections

1. Unit at a Glance

The unit follows a logical, skill-building progression from foundational symbolic chemistry to complex quantitative problem solving. You will first master the rules for writing correct formulae and balancing chemical equations, before moving to basic mass calculation that use ratio relationships, then applying the universal mole concept for standardised, scalable stoichiometric calculations.

All skills taught in this unit are directly applicable to later topics including energetics, rates of reaction, and electrolysis, where quantitative analysis is required for exam questions.

Work through the sub-topics below in order to build your stoichiometry skills incrementally:

Formulae and Chemical Equations

Learn rules for writing correct chemical formulae for ionic and covalent substances, and practice balancing symbol equations for common reactions.

★★⏱ 8 min

Relative Masses and Reacting Masses (No Mole)

Calculate relative atomic, molecular and formula masses, and solve basic reacting mass problems using equation mass ratios, no mole concept required.

★★★⏱ 10 min

The Mole and Mole Calculations

Understand the mole as the standard unit of chemical amount, and solve problems involving moles, mass, molar ratios, limiting reactants and percentage yield.

★★★★⏱ 12 min

2. Common Pitfalls

Wrong move:

Balancing equations by changing subscripts in chemical formulae instead of adding coefficients

Why:

Changing subscripts alters the chemical identity of the substance, rather than adjusting the quantity of the correct substance

Correct move:

Only adjust whole-number coefficients placed in front of formulae to balance the number of each atom on both sides of the equation

Wrong move:

Using relative atomic mass instead of relative formula/molecular mass for compounds in reacting mass calculations

Why:

This underestimates the total mass of the compound, leading to invalid ratio and final mass results

Correct move:

Sum the relative atomic masses of all atoms in a compound's formula to get its relative formula/molecular mass for all calculations

Wrong move:

Ignoring the molar ratio from a balanced equation when converting between moles of different substances

Why:

This ignores the stoichiometric relationship between reactants and products, leading to incorrect calculation of product or reactant amounts

Correct move:

Always apply the mole ratio from the balanced equation to convert between moles of one substance and moles of another in a reaction

3. Quick Reference Cheatsheet

Concept/Formula	Use Case
Balanced chemical equation	Shows the mole and mass ratios of all reactants and products in a reaction
$A_{r} = \frac{Average mass of 1 atom of element}{\frac{1}{12} mass of 1 ^{12} C atom}$	Calculate relative atomic mass of an element
$M_{r} = Sum of A_{r} values of all atoms in a formula$	Calculate relative molecular/formula mass of a compound
$Mass ratio = (M_{r} \times coefficient)_{substance 1} : (M_{r} \times coefficient)_{substance 2}$	Calculate reacting masses without using the mole concept
$Moles (mol) = \frac{Mass of substance (g)}{M _{r} of substance}$	Convert between mass and moles of a pure substance
Mole ratio = ratio of coefficients of substances in balanced equation	Relate moles of reactants to moles of products in a reaction

What's Next

Start your study of this unit with the first sub-topic on Formulae and Chemical Equations, where you will build the foundational symbolic skills required for all subsequent stoichiometry calculations. Once you have completed all three sub-topics in this unit and mastered core quantitative skills, you will move on to Unit 4 covering Bonding, Structure and Properties of Matter.