Shakir Mugasha · GCSE Combined Science · AQA
Stoichiometry & balancing chemical equations — algebraic method
Two elements · one appears once on each side · smallest whole-number solution is a = 2
Mg + O2 → MgO · 5-step process applied end-to-end
Welcome to your first worked example on the algebraic method. Today we will work through the simplest combustion reaction in GCSE chemistry: magnesium burning in oxygen to form magnesium oxide. The equation has only two elements (Mg and O), and the algebra is short — the smallest whole-number solution is a = 2. This is the same example we covered in our 2 June 2026 session, so use it to lock in the method before moving on. Work through each step, then cover the solution and try the whole thing again on your own.
Magnesium burns in oxygen to form magnesium oxide. The word equation is:
Magnesium + Oxygen → Magnesium Oxide
Written with chemical symbols (unbalanced):
The left side has 2 oxygens (in O2) but the right side has only 1 oxygen (in MgO). The equation violates the law of conservation of mass — atoms are not created or destroyed. We need to balance it.
Write a lowercase letter in front of each compound.
a Mg + b O2 → c MgOMagnesium (Mg) and Oxygen (O). Two elements total.
For Mg: 1 Mg atom on the LHS per Mg molecule, and 1 Mg atom on the RHS per MgO molecule.
a = cFor O: 2 O atoms on the LHS per O2 molecule, and 1 O atom on the RHS per MgO molecule.
2b = cThe element symbols cancel. We are left with two pure-algebra equations:
a = c 2b = cTry a = 1. Then c = 1, and b = c / 2 = 0.5. That is not a whole number, so a = 1 is wrong.
Try a = 2. Then c = 2, and b = c / 2 = 1. All whole numbers. Done.
Always verify your answer by counting every element on both sides.
| Element | LHS | RHS | Match? |
|---|---|---|---|
| Mg | 2 | 2 | ✓ |
| O | 2 | 2 | ✓ |
Every element balances. The answer is correct.
When a = 1 produces a half-integer, the smallest whole-number solution is a = 2. This is the most common pattern in simple combustion reactions.
Cover the solution with a piece of paper and try it yourself. Then slide the paper down and check your work line by line. If you can do this example cleanly in under 5 minutes, you have the method.
Worked example prepared for Shakir Mugasha — GCSE Combined Science tuition, 2 June 2026. This example walks through the algebraic method on the simple combustion of magnesium. Use it as a practice template: cover the solution, try the 5 steps yourself, then verify against the table. For the medium example (Fe + O2 → Fe3O4), see algebraic_balancing_medium_example.html.
Prepared by Dynamic Brain Hub · Human Experts - Cutting Edge AI Systems.