A student wants to find the empirical formula of a compound containing only carbon, hydrogen, and oxygen. She burns a sample of the compound completely in excess oxygen. The products are carbon dioxide and water only. Explain how the student could use the masses of carbon dioxide and water produced to determine the empirical formula of the compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The empirical formula of a compound is the simplest whole-number ratio of atoms of each element in the compound.
Model answer (5 marks)
1. Calculate moles of CO₂:
n_CO₂ = mass_CO₂ ÷ 44 g mol⁻¹
2. Moles of C = n_CO₂ (1 C per CO₂)
3. Calculate moles of H₂O:
n_H₂O = mass_H₂O ÷ 18 g mol⁻¹
4. Moles of H = 2 × n_H₂O
5. Moles of O in the compound = (mass_compound ÷ 16 g mol⁻¹) – n_C – n_H
6. Divide n_C, n_H and n_O by the smallest of the three values to obtain the simplest whole‑number ratio, which is the empirical formula.
n_CO₂ = mass_CO₂ ÷ 44 g mol⁻¹
2. Moles of C = n_CO₂ (1 C per CO₂)
3. Calculate moles of H₂O:
n_H₂O = mass_H₂O ÷ 18 g mol⁻¹
4. Moles of H = 2 × n_H₂O
5. Moles of O in the compound = (mass_compound ÷ 16 g mol⁻¹) – n_C – n_H
6. Divide n_C, n_H and n_O by the smallest of the three values to obtain the simplest whole‑number ratio, which is the empirical formula.
Examiner tips
- Show each calculation step and units; use the given relative masses (44 and 18).
- Explain the reasoning for each step – e.g. why 1 C per CO₂, why 2 H per H₂O, and how to find O moles.
- State that the final ratio must be whole numbers; round if necessary.
- Use correct UK spelling and chemical symbols.
Common mistakes
- Using the wrong molar mass for CO₂ or H₂O (e.g. 45 or 17).
- Failing to multiply the moles of H₂O by 2 to get moles of H.
- Subtracting the wrong mass (e.g. using mass of C instead of moles × 12).
Mark scheme (5 marks)
- Calculate the number of moles of carbon dioxide produced (moles = mass ÷ relative formula mass / moles = mass ÷ 44)
- Each mole of carbon dioxide contains one mole of carbon atoms, so moles of carbon = moles of CO₂
- Calculate the number of moles of water produced (moles = mass ÷ 18), and moles of hydrogen atoms = 2 × moles of water
- Find moles of oxygen by subtracting (moles of C × 16) and (moles of H × 1) from the original mass of the compound, then divide by 16
- Divide all three values of moles by the smallest value to get the simplest whole-number ratio, giving the empirical formula
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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