A chemist is investigating a sample of a compound to determine its empirical formula. The compound contains only carbon and hydrogen. The chemist finds that 3.6 g of the compound contains 3.0 g of carbon and 0.6 g of hydrogen. Explain how the empirical formula of this compound is determined from this data, and state what the empirical formula tells us about the compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A compound containing only carbon and hydrogen has a mass of 3.6 g. Of this, 3.0 g is carbon and 0.6 g is hydrogen. The relative atomic mass of carbon is 12 and the relative atomic mass of hydrogen is 1.
Model answer (5 marks)
1. Divide the mass of each element by its relative atomic mass to give the number of moles.
2. Carbon: 3.0 g ÷ 12 = 0.25 mol; Hydrogen: 0.6 g ÷ 1 = 0.60 mol.
3. Divide each mole value by the smallest value (0.25) to give the simplest whole‑number ratio.
Carbon: 0.25 ÷ 0.25 = 1; Hydrogen: 0.60 ÷ 0.25 = 2.4.
4. Multiply the ratio by 5 to convert 2.4 to a whole number: 1×5 = 5, 2.4×5 = 12.
The empirical formula is therefore C5H12.
5. The empirical formula shows the simplest whole‑number ratio of atoms of each element in the compound, indicating that for every 5 atoms of carbon there are 12 atoms of hydrogen in the simplest unit of the compound.
2. Carbon: 3.0 g ÷ 12 = 0.25 mol; Hydrogen: 0.6 g ÷ 1 = 0.60 mol.
3. Divide each mole value by the smallest value (0.25) to give the simplest whole‑number ratio.
Carbon: 0.25 ÷ 0.25 = 1; Hydrogen: 0.60 ÷ 0.25 = 2.4.
4. Multiply the ratio by 5 to convert 2.4 to a whole number: 1×5 = 5, 2.4×5 = 12.
The empirical formula is therefore C5H12.
5. The empirical formula shows the simplest whole‑number ratio of atoms of each element in the compound, indicating that for every 5 atoms of carbon there are 12 atoms of hydrogen in the simplest unit of the compound.
Examiner tips
- Use the command word ‘explain’ – show each calculation step and the reasoning behind it.
- State the empirical formula clearly and explain its meaning in one sentence.
- Show the conversion of the fractional ratio to whole numbers.
Mark scheme (5 marks)
- Divide the mass of each element by its relative atomic mass to find the number of moles (ratio) of each element
- Carbon gives 3.0 ÷ 12 = 0.25 (moles) and hydrogen gives 0.6 ÷ 1 = 0.6 (moles)
- Divide both values by the smallest value (0.25) to obtain the simplest whole number ratio
- Correct empirical formula stated as CH2 (ratio C:H = 1:2.4, multiply by 5 gives C5H12, but simplest whole number ratio from 1:2.4 → multiply by 5 gives C5:H12, so empirical formula is C5H12) — accept CH2 if ratio correctly derived as 1:2.4 rounded/multiplied to smallest integers giving C5H12
- The empirical formula shows the simplest whole number ratio of atoms of each element in the compound
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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