Balancing Chemical Equations — Drills · Grade 11 Science

Chapter: Reactions, Moles & Composition

What these drills covers

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

6 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

The large number written in front of a chemical formula is called the…
  1. coefficient
  2. exponent
  3. isotope
  4. subscript
Answer and explanation

coefficient

The number in front of a formula is the coefficient; it tells how many units of that substance react.

Problem 2

In the balanced equation 2H₂ + O₂ → 2H₂O, how many oxygen atoms are on the reactant side?
Answer and explanation

2 atoms

O₂ contains 2 oxygen atoms, matching the 2 oxygen atoms in 2H₂O on the product side.

Problem 3

Which coefficients balance ___N₂ + ___H₂ → ___NH₃?
  1. 1, 3, 2
  2. 1, 1, 1
  3. 2, 3, 2
  4. 1, 2, 2
Answer and explanation

1, 3, 2

N₂ + 3H₂ → 2NH₃ gives 2 N and 6 H atoms on both sides.

Problem 4

Balance ___Mg + ___O₂ → ___MgO. What coefficient belongs in front of MgO?
Answer and explanation

2

2Mg + O₂ → 2MgO balances 2 Mg and 2 O on each side, so the coefficient of MgO is 2.

Problem 5

A student tries to balance H₂ + O₂ → H₂O by rewriting the product as H₂O₂. Explain why this is not allowed.
Answer and explanation

Changing the subscript turns water (H₂O) into hydrogen peroxide (H₂O₂), a completely different substance. You must balance by changing coefficients, which only changes how many molecules you have, not their identity.

Problem 6

Balance the combustion of ethane using smallest whole numbers: ___C₂H₆ + ___O₂ → ___CO₂ + ___H₂O. What is the coefficient of O₂?
Answer and explanation

7

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O: carbon 4 = 4, hydrogen 12 = 12, oxygen 14 = 14. The coefficient of O₂ is 7.

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