Reacting Masses

This lesson covers how to calculate reacting masses using balanced equations and moles, as required by the AQA GCSE Combined Science Trilogy specification (8464). This is one of the most important calculation skills in GCSE chemistry.

---

The Method for Reacting Mass Calculations

flowchart TD
    A["1. Write the balanced equation"] --> B["2. Calculate the Mr of the\nsubstances you need"]
    B --> C["3. Find the moles of the\nsubstance you know"]
    C --> D["4. Use the mole ratio from\nthe balanced equation"]
    D --> E["5. Calculate the mass of the\nsubstance you want"]
    style A fill:#3b82f6,color:#fff,stroke:#2563eb
    style C fill:#f59e0b,color:#000,stroke:#d97706
    style E fill:#22c55e,color:#fff,stroke:#16a34a

Step-by-Step

1. Write the balanced equation.
2. Calculate the $M_r$Mr​ of the relevant substances.
3. Find the moles of the substance whose mass you know: $\text{moles} = \dfrac{\text{mass}}{M_r}$moles=Mr​mass​
4. Use the mole ratio from the balanced equation to find the moles of the substance you want.
5. Calculate the mass of the substance you want: $\text{mass} = \text{moles} \times M_r$mass=moles×Mr​

---

Worked Examples

Example 1 — Magnesium and Oxygen

Question: What mass of magnesium oxide (MgO) is produced when 4.8 g of magnesium reacts completely with oxygen?

$2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$2Mg(s)+O2​(g)→2MgO(s)