Worked example
In 2H₂ + O₂ → 2H₂O, how many hydrogen atoms are on the left?
Answer & why
Answer: 4
The coefficient 2 multiplies the 2 hydrogen atoms in each H₂ molecule.
Grade 8 · Science
Count atoms in chemical equations
Use coefficients and subscripts to check conservation of each element in a simple equation.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
Which coefficient before O₂ balances 2Mg + ___O₂ → 2MgO?
- 2
- 4
- 1
- 3
A hint
A coefficient multiplies every atom in the formula; compare each element on both sides.
Answer & why
Answer: 1
One O₂ supplies two oxygen atoms for two MgO formula units.
Question 2
In CO₂, how many oxygen atoms does the subscript show per molecule?
- 1
- 0
- 2
- 3
A hint
A coefficient multiplies every atom in the formula; compare each element on both sides.
Answer & why
Answer: 2
The subscript 2 belongs to oxygen in CO₂.
Question 3
In 3CO₂, how many carbon atoms are represented?
- 1
- 3
- 6
- 2
A hint
A coefficient multiplies every atom in the formula; compare each element on both sides.
Answer & why
Answer: 3
Each CO₂ has one carbon atom; the coefficient gives three molecules.
Question 4
A proposed equation has 4 oxygen atoms on the left and 2 on the right, with no other substances listed. Is oxygen balanced?
- Yes; any arrow guarantees balance.
- Yes; oxygen atoms may be ignored.
- No; equations never conserve atoms.
- No; the oxygen counts differ.
A hint
A coefficient multiplies every atom in the formula; compare each element on both sides.
Answer & why
Answer: No; the oxygen counts differ.
Conservation requires equal counts of each element across the complete equation.
Question 5
In 2H₂ + O₂ → 2H₂O, how many hydrogen atoms are on the left?
- 4
- 8
- 6
- 2
A hint
A coefficient multiplies every atom in the formula; compare each element on both sides.
Answer & why
Answer: 4
The coefficient 2 multiplies the 2 hydrogen atoms in each H₂ molecule.
Question 6
In 2H₂ + O₂ → 2H₂O, how many oxygen atoms are on the right?
- 1
- 2
- 0
- 4
A hint
A coefficient multiplies every atom in the formula; compare each element on both sides.
Answer & why
Answer: 2
Two H₂O molecules each contain one oxygen atom.