Sodium carbonate has the formula Na₂CO₃. State the number of atoms of each element present in one formula unit of sodium carbonate, and use this information to explain the difference between an empirical formula and a molecular formula.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium carbonate, Na₂CO₃, is a white solid used in glass manufacturing. Its empirical formula is NaCO₃ and its molecular formula is Na₂CO₃.
Model answer (4 marks)
Sodium carbonate contains 2 sodium atoms, 1 carbon atom and 3 oxygen atoms per formula unit.
The empirical formula gives the simplest whole‑number ratio of atoms of each element in a compound. For Na₂CO₃ the empirical formula is NaCO₃, which cannot be simplified further.
The molecular formula shows the actual number of atoms of each element in one molecule (or formula unit) of the compound. For Na₂CO₃ the molecular formula contains twice as many sodium atoms as the empirical formula, showing the real stoichiometry of the compound.
The empirical formula gives the simplest whole‑number ratio of atoms of each element in a compound. For Na₂CO₃ the empirical formula is NaCO₃, which cannot be simplified further.
The molecular formula shows the actual number of atoms of each element in one molecule (or formula unit) of the compound. For Na₂CO₃ the molecular formula contains twice as many sodium atoms as the empirical formula, showing the real stoichiometry of the compound.
Examiner tips
- State the exact atom counts first to secure 1 mark. Explain the empirical formula as a simplest ratio – 1 mark. Explain the molecular formula as the real atom count – 1 mark. Show the relationship (Na₂CO₃ vs NaCO₃) – 1 mark.
Common mistakes
- Giving the wrong atom counts (e.g. 1 Na instead of 2). Confusing the empirical formula with the molecular formula. Omitting the explanation of why the empirical formula cannot be simplified further.
Mark scheme (4 marks)
- Sodium carbonate contains 2 sodium atoms, 1 carbon atom, and 3 oxygen atoms (per formula unit)
- The empirical formula shows the simplest whole number ratio of atoms of each element in a compound
- The molecular formula shows the actual number of atoms of each element in one molecule / formula unit of the compound
- For Na₂CO₃, the empirical formula (NaCO₃) cannot be simplified further to a simpler whole number ratio / the molecular formula (Na₂CO₃) shows twice as many atoms as the empirical formula
Key terms in this question
empirical formula · molecular formula · atom
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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