Balancing Chemical Equations — Practice · Grade 11 Science

Chapter: Reactions, Moles & Composition

What this practice covers

By the end of Balancing Chemical Equations you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: chemical equation, coefficient, subscript, balanced equation, law of conservation of mass.

Problem 1

When balancing an equation, why may you change coefficients but never subscripts?
  1. Subscripts must always be 1
  2. There is no real difference between them
  3. Coefficients are easier to write
  4. Changing a subscript would change the substance into a different chemical, while a coefficient only changes how many units you have
Answer and explanation

Changing a subscript would change the substance into a different chemical, while a coefficient only changes how many units you have

A subscript defines a substance's identity (H₂O vs H₂O₂), so only coefficients — the 'how many' — can be adjusted.

Problem 2

In the balanced equation N₂ + 3H₂ → 2NH₃, how many hydrogen atoms are on each side?
Answer and explanation

6 atoms

Left: 3H₂ = 3 × 2 = 6 H. Right: 2NH₃ = 2 × 3 = 6 H. They match.

Problem 3

Which set of coefficients correctly balances ___Al + ___O₂ → ___Al₂O₃?
  1. 1, 1, 1
  2. 2, 3, 1
  3. 4, 3, 2
  4. 2, 1, 2
Answer and explanation

4, 3, 2

4Al + 3O₂ → 2Al₂O₃ gives 4 Al and 6 O atoms on both sides — the smallest whole-number balance.

Problem 4

Balance the decomposition ___KClO₃ → ___KCl + ___O₂. What coefficient belongs in front of O₂?
Answer and explanation

3

2KClO₃ → 2KCl + 3O₂ balances: K 2 = 2, Cl 2 = 2, O 6 = 6. The coefficient on O₂ is 3.

Problem 5

Explain how balancing a chemical equation is a direct application of the law of conservation of mass.
Answer and explanation

Conservation of mass says atoms are only rearranged, never created or destroyed, so the same number of each atom must appear on both sides. Balancing forces the atom counts to match, which guarantees the total mass of the reactants equals the total mass of the products.

Problem 6

Gasoline burns by the equation 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O. How many molecules of O₂ are required to burn 2 molecules of octane (C₈H₁₈)?
Answer and explanation

25 molecules

The balanced coefficient on O₂ is 25: two octane molecules need 25 oxygen molecules for complete combustion.

Problem 7

Balance Al + O₂ → Al₂O₃. Show the reasoning, not just the conclusion.
Hints

Balanced coefficients are ratios, not proof of complete reaction under every condition.

Answer and explanation

4Al + 3O₂ → 2Al₂O₃; Al=4 and O=6 on both sides.

Problem 8

Why can a balanced reaction have unequal totals of molecules on its two sides?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Atoms are conserved, but grouping those atoms into different molecules changes molecule counts.

Problem 9

Smallest whole-number coefficients are a reporting conventionSmallest whole-number coefficients are a reporting convention4H₂ + 2O₂ → 4H₂O2H₂ + O₂ → 2H₂ODivide every coefficient by their common factor of 2.The physical ratio is unchanged.
Original coefficient-scaling comparison, not a claim that a reaction’s mechanism requires exactly this many simultaneous molecular collisions.
Interpret the supplied visual. Why can 4:2:4 and 2:1:2 describe the same reaction ratio while an associated heat value must be scaled?
Answer and explanation

The ratios are equivalent, but the thermochemical amount basis changes. Heat is extensive and must change with the equation amount.

Multiplying all coefficients by the same positive factor preserves the stoichiometric ratio and conservation. For an ordinary school balancing answer, report the smallest whole-number coefficients. A thermochemical equation may deliberately use a different scale, including a fraction, to define an energy change per specified amount. The equation’s normalization matters when relating it to moles or reaction enthalpy; reducing one coefficient alone would break the composition account. State the convention and scale the full equation consistently.

The ratios are equivalent, but the thermochemical amount basis changes. Heat is extensive and must change with the equation amount.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

Balance C₃H₈ + O₂ → CO₂ + H₂O using the smallest whole-number coefficients, then show the atom counts.
Answer and explanation

C₃H₈ +5O₂ →3CO₂ +4H₂O. Counts are C:3, H:8, O:10 on each side. Formula subscripts remain unchanged.

C₃H₈ +5O₂ →3CO₂ +4H₂O. Counts are C:3, H:8, O:10 on each side. Formula subscripts remain unchanged.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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