# Stoichiometry

> CIE IGCSE Chemistry · CIE IGCSE Chemistry (0620)
> Source: https://www.owlsprep.com/study/cie-0620-u3-overview/
> Weight: 12-15% of total exam, across MCQ, structured and extended response sections

This unit covers core quantitative chemistry principles, including symbolic reaction representation, mass ratio calculations, and the mole concept, which underpin all quantitative chemistry problems across the CIE IGCSE Chemistry syllabus.

**Prerequisites:** [Atomic Structure and the Periodic Table](https://www.owlsprep.com/study/cie-0620-u2-overview/); [Elements, Compounds and Mixtures](https://www.owlsprep.com/study/cie-0620-u1-overview/)

## Learning objectives

- Write correct chemical formulae for ionic and covalent substances, and balance full symbol equations for chemical reactions
- Calculate relative atomic, molecular and formula masses for elements and compounds
- Solve reacting mass problems using mass ratios from balanced equations, with and without the mole concept
- Apply the mole concept to solve problems involving mass, molar ratios, limiting reactants and percentage yield

## 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](https://www.owlsprep.com/study/cie-0620-u3-formulae-and-chemical-equations/) — Learn rules for writing correct chemical formulae for ionic and covalent substances, and practice balancing symbol equations for common reactions.
- [Relative Masses and Reacting Masses (No Mole)](https://www.owlsprep.com/study/cie-0620-u3-relative-masses-and-reacting-masses/) — Calculate relative atomic, molecular and formula masses, and solve basic reacting mass problems using equation mass ratios, no mole concept required.
- [The Mole and Mole Calculations](https://www.owlsprep.com/study/cie-0620-u3-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.

## Common pitfalls

- **Wrong:** Balancing equations by changing subscripts in chemical formulae instead of adding coefficients
  - Why it fails: Changing subscripts alters the chemical identity of the substance, rather than adjusting the quantity of the correct substance
  - Correct: Only adjust whole-number coefficients placed in front of formulae to balance the number of each atom on both sides of the equation
- **Wrong:** Using relative atomic mass instead of relative formula/molecular mass for compounds in reacting mass calculations
  - Why it fails: This underestimates the total mass of the compound, leading to invalid ratio and final mass results
  - Correct: Sum the relative atomic masses of all atoms in a compound's formula to get its relative formula/molecular mass for all calculations
- **Wrong:** Ignoring the molar ratio from a balanced equation when converting between moles of different substances
  - Why it fails: This ignores the stoichiometric relationship between reactants and products, leading to incorrect calculation of product or reactant amounts
  - Correct: Always apply the mole ratio from the balanced equation to convert between moles of one substance and moles of another in a reaction

## Cheatsheet

| Concept/Formula | Use Case |
| --- | --- |
| Balanced chemical equation | Shows the mole and mass ratios of all reactants and products in a reaction |
| $A_r = \frac{\text{Average mass of 1 atom of element}}{\frac{1}{12} \text{ mass of 1 } ^{12}\text{C atom}}$ | Calculate relative atomic mass of an element |
| $M_r = \text{Sum of } A_r \text{ values of all atoms in a formula}$ | Calculate relative molecular/formula mass of a compound |
| $\text{Mass ratio} = (M_r \times \text{coefficient})_\text{substance 1} : (M_r \times \text{coefficient})_\text{substance 2}$ | Calculate reacting masses without using the mole concept |
| $\text{Moles (mol)} = \frac{\text{Mass of substance (g)}}{M_r \text{ of substance}}$ | 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.

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