IGCSE Chemistry Stoichiometry Notes
Stoichiometry is the arithmetic of chemistry — using balanced equations and moles to work out how much of each substance reacts or is made.
Key concepts
- Balanced equation
- Same number of each atom on both sides; only add coefficients, never change formulae.
- Relative formula mass (Mᵣ)
- Sum of the Aᵣ values of all atoms in a formula unit.
- Mole
- 6.02 × 10²³ particles; the amount of substance whose mass in grams equals Mᵣ.
- Mole triangle
- moles = mass ÷ Mᵣ; mass = moles × Mᵣ; Mᵣ = mass ÷ moles.
- Balancing goes: metals → non-metals → hydrogen → oxygen.
- 1 mole of any gas occupies 24 dm³ at room temperature and pressure.
- Concentration (mol/dm³) = moles ÷ volume in dm³.
- Percentage yield = (actual ÷ theoretical) × 100%.
Worked example
Calculate the mass of magnesium oxide produced when 4.8 g of magnesium fully reacts with oxygen. (Aᵣ: Mg = 24, O = 16)
- 1Equation: 2Mg + O₂ → 2MgO
- 2Moles of Mg = 4.8 ÷ 24 = 0.20 mol
- 3Mole ratio Mg : MgO = 1 : 1, so moles of MgO = 0.20 mol
- 4Mass of MgO = 0.20 × 40 = 8.0 g
Answer: 8.0 g of MgO
Common mistakes
✗ Changing the small subscripts inside a formula to balance an equation.
✓ Only add coefficients in front of formulae; never edit subscripts.
✗ Using mass instead of moles in ratios.
✓ Convert mass → moles first, use ratio, then convert back to mass.
✗ Forgetting units for concentration or volume.
✓ Always check dm³ and mol/dm³ — 1000 cm³ = 1 dm³.
Quick check
Try these, then reveal the answers.
How many moles are in 22 g of CO₂ (Mᵣ = 44)?Reveal answer
0.5 mol
Balance: __H₂ + __O₂ → __H₂OReveal answer
2H₂ + O₂ → 2H₂O
What volume does 0.25 mol of a gas occupy at RTP?Reveal answer
6 dm³ (0.25 × 24).
Exam tips
- Write the balanced equation first — most marks depend on it.
- Layout: equation → moles of what you know → ratio → moles of what you want → answer with units.
- Round only at the very end; keep at least 3 significant figures during working.
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Frequently asked questions
What is a mole?+
The amount of substance containing 6.02 × 10²³ particles — the same number of atoms as in exactly 12 g of carbon-12.
Why balance chemical equations?+
Because atoms are neither created nor destroyed in a reaction; both sides must have the same atoms.
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