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Balancing chemical equations practice
A balanced equation conserves the number of atoms of each element. Change coefficients in front of formulas, never subscripts inside them.
By ExamCat · Original study lesson · Published September 24, 2026 · Updated September 25, 2026
Understand the method
Count each element on both sides, including the multiplier outside a formula. Treat unchanged polyatomic groups as units when useful, but finish with a complete atom check. Reduce whole-number coefficients to their smallest common ratio.
Worked example, step by step
Balance the combustion equation C₃H₈ + O₂ → CO₂ + H₂O.
- Balance carbon first: three carbon atoms require 3CO₂.
- Balance hydrogen: eight hydrogen atoms require 4H₂O.
- Count product oxygen: 3 × 2 + 4 × 1 = 10 atoms. Use 5O₂.
- Verify C₃H₈ + 5O₂ → 3CO₂ + 4H₂O: both sides contain 3 C, 8 H and 10 O atoms.
A mistake to watch for
Changing H₂O to H₂O₂ would create a different substance rather than balance the original reaction. An equation can also be mathematically balanced without proving that a reaction actually occurs under every condition.
Try it yourself: 12 questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. Balance Al + O₂ → Al₂O₃.
Show answer and explanation
4Al + 3O₂ → 2Al₂O₃. Each side has four aluminum atoms and six oxygen atoms.
2. Balance H₂ + Cl₂ → HCl.
Show answer and explanation
H₂ + Cl₂ → 2HCl. Two product molecules account for both atoms from each diatomic reactant.
3. Are coefficients 2, 10, 6, 8 wrong for the propane example?
Show answer and explanation
They conserve atoms but are not the smallest whole-number ratio. Divide all four coefficients by two; never reduce just one.
4. Balance Mg + O₂ → MgO.
Show answer and explanation
2Mg + O₂ → 2MgO. Oxygen arrives as pairs, so make two MgO units, then supply two Mg atoms. Both sides contain 2 Mg and 2 O.
5. Balance N₂ + H₂ → NH₃.
Show answer and explanation
N₂ + 3H₂ → 2NH₃. Two NH₃ molecules contain 2 N and 6 H atoms, requiring one N₂ and three H₂ molecules.
6. Balance KClO₃ → KCl + O₂.
Show answer and explanation
2KClO₃ → 2KCl + 3O₂. The smallest shared oxygen count for groups of three and two is six. The coefficient two on KClO₃ also requires two KCl.
7. Balance Fe + O₂ → Fe₂O₃.
Show answer and explanation
4Fe + 3O₂ → 2Fe₂O₃. Start with six oxygen atoms on each side, then balance the four iron atoms.
8. Balance NaOH + H₂SO₄ → Na₂SO₄ + H₂O.
Show answer and explanation
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. Two NaOH supply the two Na atoms. The left has four H atoms, requiring two water molecules. Both sides have six O atoms.
9. Balance CaCl₂ + Na₃PO₄ → Ca₃(PO₄)₂ + NaCl.
Show answer and explanation
3CaCl₂ + 2Na₃PO₄ → Ca₃(PO₄)₂ + 6NaCl. Keep each phosphate group together: one product unit needs two PO₄ groups and three Ca atoms. That leaves six Na and six Cl atoms for six NaCl units.
10. Balance C₂H₆ + O₂ → CO₂ + H₂O using whole numbers.
Show answer and explanation
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. One ethane gives two CO₂ and three H₂O, requiring 7/2 O₂. Multiply every coefficient by two. Check 4 C, 12 H and 14 O on each side.
11. Balance C₂H₅OH + O₂ → CO₂ + H₂O.
Show answer and explanation
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Ethanol already supplies one oxygen atom. Products contain seven O atoms total, so only six more must come from three O₂ molecules.
12. A student writes 2H₂ + O₂ → 2H₂O₂. What is wrong, and how do you fix it if the product is water?
Show answer and explanation
The left contains two oxygen atoms, but the right contains four. More importantly, H₂O₂ is hydrogen peroxide, not water. Restore the given water formula: 2H₂ + O₂ → 2H₂O. Subscripts identify substances; coefficients count them.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Chemistry 2e: Writing and Balancing Chemical Equations provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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