Calcium reacts with oxygen to form calcium oxide. A student claims that the mass of calcium oxide produced must equal the total mass of calcium and oxygen that reacted. Explain why the student is correct, and describe what an empirical formula tells a chemist about a compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants (law of conservation of mass). An empirical formula gives the simplest whole‑number ratio of atoms of each element in a compound. For calcium oxide the empirical formula is CaO, showing a 1:1 ratio of calcium to oxygen atoms. The empirical formula may differ from the molecular formula, which shows the actual number of atoms present in a molecule.
Examiner tips
- Use the law of conservation of mass to justify the mass equality
- State that an empirical formula is the simplest whole‑number ratio
- Give the empirical formula of CaO and note the 1:1 ratio
- Mention that the empirical formula can differ from the molecular formula
Common mistakes
- Confusing empirical formula with molecular formula
- Failing to mention the 1:1 ratio for CaO
- Ignoring the conservation of mass principle
Mark scheme (5 marks)
- No atoms are lost or made during a chemical reaction
- Therefore the mass of the products equals the mass of the reactants (law of conservation of mass)
- An empirical formula gives the simplest whole number ratio of atoms of each element in a compound
- The empirical formula of calcium oxide is CaO, showing a 1:1 ratio of calcium to oxygen atoms
- The empirical formula may differ from the molecular formula, which shows the actual number of atoms present in a molecule
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
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- Decode the mark scheme abbreviations →
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