Chemistry equations and calculations account for over 40% of WAEC Chemistry marks, yet most students lose unnecessary points due to simple balancing errors, wrong stoichiometry, or formula confusion.
This comprehensive guide breaks down every equation type you'll encounter in WAEC Chemistry 2026, with step-by-step methods, common pitfalls, and time-saving shortcuts that top-scoring students use.
🎯 Understanding Chemical Equations Basics
What Examiners Look For:
- Correct chemical formulas - One wrong subscript = zero marks
- Balanced equations - Atoms must be equal on both sides
- State symbols - (s), (l), (g), (aq) are often required
- Reaction conditions - Heat, catalyst, temperature when specified
💡 Pro Tip from High Scorers:
Write all formulas FIRST, verify them, THEN balance. Trying to balance while writing formulas leads to costly mistakes.
âš–ï¸ Mastering Equation Balancing
The 4-Step Balancing Method:
Step 1: Write Skeletal Equation
Example: Combustion of methane
CH₄ + O₂ → CO₂ + H₂O
Step 2: Count Atoms
- Left: C=1, H=4, O=2
- Right: C=1, H=2, O=3
Step 3: Balance One Element at a Time
Start with most complex molecule (usually compounds, not elements):
CH₄ + O₂ → CO₂ + 2H₂O
Step 4: Balance Remaining Elements
CH₄ + 2O₂ → CO₂ + 2H₂O
✓ Verify: C=1/1, H=4/4, O=4/4 ✓
âš ï¸ Common Balancing Mistakes:
- Changing subscripts instead of coefficients (NEVER do this!)
- Forgetting diatomic molecules (Hâ‚‚, Oâ‚‚, Nâ‚‚, Fâ‚‚, Clâ‚‚, Brâ‚‚, Iâ‚‚)
- Not using fractions when needed (you can always multiply through later)
- Balancing polyatomic ions separately when they appear on both sides
📠Stoichiometry Calculations
Mole Concept Formula Sheet:
Number of moles (n) =
- Mass (g) ÷ Molar mass (g/mol)
- Volume (dm³) × Concentration (mol/dm³)
- Volume of gas (dm³) ÷ 22.4 (at STP) or 24 (at RTP)
- Number of particles ÷ Avogadro's number (6.02 × 10²³)
Worked Example: Mass-Mass Calculations
Question: What mass of magnesium oxide is produced when 6g of magnesium burns completely in oxygen?
[Mg = 24, O = 16]
Solution:
- Write balanced equation:
2Mg + O₂ → 2MgO - Calculate moles of Mg:
n = mass ÷ molar mass = 6g ÷ 24g/mol = 0.25 mol - Use mole ratio:
2 mol Mg produces 2 mol MgO
∴ 0.25 mol Mg produces 0.25 mol MgO - Calculate mass of MgO:
Molar mass MgO = 24 + 16 = 40 g/mol
Mass = n × molar mass = 0.25 × 40 = 10g
Answer: 10g of MgO
🔬 Important WAEC Chemistry Equations by Topic
1. Acids, Bases & Salts
Acid + Metal → Salt + Hydrogen
Zn + H₂SO₄ → ZnSO₄ + H₂
Acid + Base → Salt + Water
HCl + NaOH → NaCl + H₂O
Acid + Carbonate → Salt + Water + CO₂
2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
2. Redox Reactions
Displacement reactions:
Fe + CuSO₄ → FeSO₄ + Cu
(More reactive Fe displaces less reactive Cu)
Reduction of metal oxides:
CuO + H₂ → Cu + H₂O
Fe₂O₃ + 3CO → 2Fe + 3CO₂
3. Organic Chemistry
Combustion of alkanes:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Fermentation:
C₆Hâ‚â‚‚O₆ → 2Câ‚‚Hâ‚…OH + 2COâ‚‚
Esterification:
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
4. Electrolysis
Electrolysis of brine:
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g)
At cathode: 2H⺠+ 2e⻠→ H₂
At anode: 2Cl⻠→ Clâ‚‚ + 2eâ»
âš¡ Time-Saving Calculation Shortcuts
Shortcut 1: Mass Percentage Composition
Instead of calculating moles:
% element = (Mass of element in formula / Total molar mass) × 100
Shortcut 2: Quick Mole Ratios
For limiting reagent problems, divide mass by (molar mass × coefficient)
The smallest result is the limiting reagent
Shortcut 3: Gas Volume Calculations
At room temp/pressure (RTP): 1 mole = 24 dm³
At standard temp/pressure (STP): 1 mole = 22.4 dm³
Remember: WAEC usually uses RTP unless stated otherwise
📠Practice Questions to Master
Essential Equation Practice:
- Balance: __Fe + __O₂ → __Fe₃O₄
- Calculate mass of CO₂ from burning 10g of C₂H₆ [C=12, H=1, O=16]
- What volume of Hâ‚‚ at RTP is produced from 0.5 mol of Zn in HCl?
- Write equation for reaction between sulfuric acid and sodium hydroxide
- Calculate % of nitrogen in NH₄NO₃ [N=14, H=1, O=16]
Practice these daily until you can solve them in under 2 minutes each!
✅ Exam Day Strategy
Before Writing Any Equation:
- Read the question twice - identify what's being asked
- Write known formulas on scratch paper
- Check your periodic table for atomic masses
- Show ALL working - even if answer is wrong, method marks count
- Double-check balancing before moving to calculations
- Include units in all final answers
âš ï¸ Never:
- Skip units (you'll lose marks even with correct numbers)
- Round off until final answer (keep 3-4 sig figs during calculation)
- Forget state symbols when question asks for them
- Use wrong molar masses (always check atomic numbers)
🎯 Final Preparation Checklist
Week Before Exam:
- ✓ Memorize formulas of common compounds (acids, bases, salts)
- ✓ Practice balancing 20 equations daily
- ✓ Solve 5 stoichiometry problems under timed conditions
- ✓ Review organic chemistry reaction pathways
- ✓ Test yourself on mole concept formulas
- ✓ Master ionic and half-equations for electrolysis
🚀 Ready to Practice Chemistry Equations?
Test your equation skills with our interactive WAEC Chemistry practice questions. Get instant feedback and detailed solutions!
Practice Chemistry Now