Reacting Masses

This lesson shows you how to use balanced equations and moles to calculate the masses of reactants and products in a chemical reaction, as required by the Edexcel GCSE Combined Science specification (1SC0). This is one of the most important calculation skills in GCSE chemistry.

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The Principle

A balanced equation tells you the ratio of moles in which substances react and are produced. You can use this ratio, along with the moles formula, to convert between masses.

The Step-by-Step Method

flowchart LR
    A["Write balanced\nequation"] --> B["Calculate M_r\nof substances"]
    B --> C["Convert given\nmass → moles"]
    C --> D["Apply mole\nratio"]
    D --> E["Convert\nmoles → mass"]
    style A fill:#6366f1,color:#fff,stroke:#4f46e5
    style B fill:#8b5cf6,color:#fff,stroke:#7c3aed
    style C fill:#a855f7,color:#fff,stroke:#9333ea
    style D fill:#c084fc,color:#000,stroke:#a855f7
    style E fill:#e879f9,color:#000,stroke:#d946ef

1. Write the balanced equation.
2. Calculate the $M_r$Mr​ of the substances you need.
3. Convert the given mass to moles using $\text{moles} = \dfrac{\text{mass}}{M_r}$moles=Mr​mass​.
4. Use the ratio from the balanced equation to find moles of the desired substance.
5. Convert moles back to mass using $\text{mass} = \text{moles} \times M_r$mass=moles×Mr​.

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Worked Examples

Example 1 — Magnesium and Oxygen