Balancing Chemical Equations — Practice · Grade 8 Science

Chapter: Chemistry

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, model, evidence, system boundary, assumption.

Problem 1

In the balanced equation 2H₂ + O₂ → 2H₂O, how many total hydrogen atoms are on the reactant side?
  1. 2
  2. 4
  3. 6
  4. 8
Answer and explanation

4

2 molecules of H₂ each have 2 hydrogen atoms: 2 × 2 = 4 hydrogen atoms.

Problem 2

In the equation 2Mg + O₂ → 2MgO, how many oxygen atoms are on the product side?
Answer and explanation

2 atoms

2 MgO means 2 formula units, each with 1 oxygen atom, so there are 2 oxygen atoms total — matching the 2 oxygen atoms in O₂ on the reactant side.

Problem 3

Which coefficients correctly balance this equation? ___N₂ + ___H₂ → 2NH₃
  1. 1 and 1
  2. 1 and 3
  3. 2 and 3
  4. 1 and 2
Answer and explanation

1 and 3

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

Problem 4

Explain why you can change coefficients but not subscripts when balancing an equation.
Answer and explanation

Changing a subscript would change the identity of the substance itself (turning it into a different compound), but a coefficient just changes how many molecules you have of the same substance — the atoms and identity stay the same.

Problem 5

The equation CH₄ + 2O₂ → CO₂ + 2H₂O is balanced. How many total oxygen atoms are on the reactant side?
Answer and explanation

4 atoms

2 molecules of O₂ each have 2 oxygen atoms: 2 × 2 = 4 oxygen atoms.

Problem 6

Balance H₂ + O₂ → H₂O and verify each element. Explain the reasoning in your own words.
Answer and explanation

2H₂ + O₂ → 2H₂O; H=4 and O=2 on each side.

Put 2 before H₂O to give two oxygen atoms on the right.

Put 2 before H₂ so both sides contain four hydrogen atoms.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Balance Mg + O₂ → MgO and verify atom counts. Show the reasoning, not just the conclusion.
Hints

Totals of molecules differ, but each element's atom count matches.

Answer and explanation

2Mg + O₂ → 2MgO. Both sides contain 2 Mg and 2 O atoms.

Problem 8

Does 2H₂ + O₂ → 2H₂O require equal molecule counts on both sides? Explain.
Hints

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

Answer and explanation

No. There are 3 reactant molecules and 2 product molecules in the ratio, but 4 H and 2 O atoms on each side.

Problem 9

Balance counts by changing whole-particle coefficientsBalance counts by changing whole-particle coefficientsUnbalanced: H₂ + O₂ → H₂OLeft: H 2, O 2. Right: H 2, O 1.Balanced: 2H₂ + O₂ → 2H₂OLeft: H 4, O 2. Right: H 4, O 2.A coefficient counts whole molecules. Changing H₂O to H₂O₂ changes identity.
The first specified water equation has unequal oxygen counts. The balanced form uses coefficients 2, 1 and 2 to preserve four H atoms and two O atoms. Subscripts remain unchanged because they describe each substance. An equation arrow describes the chemical account; it is not a physical force or motion arrow.
For this unit’s illustrated case, why must adding 2 before H₂O also be followed by changing the H₂ coefficient?
Answer and explanation

Two H₂O molecules contain four H atoms, so two H₂ reactant molecules are needed to match them. The O₂ molecule already supplies the two O atoms.

A coefficient multiplies every atom in a formula. Recount all elements after each change, including an element that was previously balanced. This prevents solving the oxygen mismatch by accidentally creating a hydrogen mismatch.

Two H₂O molecules contain four H atoms, so two H₂ reactant molecules are needed to match them. The O₂ molecule already supplies the two O atoms.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 10

For the supplied reaction N₂ + H₂ → NH₃, keep all formulas unchanged and find the simplest whole-number coefficients. Check both element counts.
Answer and explanation

N₂ + 3H₂ → 2NH₃. Both sides contain 2 N atoms and 6 H atoms.

Match two nitrogen atoms using 2NH₃.

That product count needs six H atoms, supplied by 3H₂.

N₂ + 3H₂ → 2NH₃. Both sides contain 2 N atoms and 6 H atoms.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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