A student wants to find the empirical formula of a compound containing only carbon and hydrogen. The compound is found to contain 75% carbon by mass. The rest of the mass is hydrogen. Use this information to determine the empirical formula of the compound. Show your reasoning clearly, stating the steps you would take.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A compound contains only carbon and hydrogen. Analysis shows that 75% of its mass is carbon and the remaining 25% is hydrogen. The relative atomic mass of carbon is 12 and the relative atomic mass of hydrogen is 1.
Model answer (5 marks)
Assume a 100 g sample so that the percentages are masses in grams: 75 g C and 25 g H.
Divide each mass by the relative atomic mass to give moles:
- C: 75 g ÷ 12 g mol⁻¹ = 6.25 mol
- H: 25 g ÷ 1 g mol⁻¹ = 25 mol
Divide both by the smallest value (6.25 mol) to obtain the simplest whole‑number ratio:
- C: 6.25 ÷ 6.25 = 1
- H: 25 ÷ 6.25 = 4
Thus the empirical formula is CH₄.
Divide each mass by the relative atomic mass to give moles:
- C: 75 g ÷ 12 g mol⁻¹ = 6.25 mol
- H: 25 g ÷ 1 g mol⁻¹ = 25 mol
Divide both by the smallest value (6.25 mol) to obtain the simplest whole‑number ratio:
- C: 6.25 ÷ 6.25 = 1
- H: 25 ÷ 6.25 = 4
Thus the empirical formula is CH₄.
Examiner tips
- State the 100 g assumption first to justify using percentages as masses.
- Show the division by atomic masses explicitly and keep units.
- Divide by the smallest mole value to get whole numbers.
- Write the final formula clearly as CH₄.
Common mistakes
- Using the wrong atomic mass for hydrogen (e.g. 1.008) and getting a non‑whole‑number ratio.
- Failing to divide by the smallest mole value, leaving 1 : 4 as 1 : 4 but not simplifying.
- Writing the empirical formula as CH₄ without showing the calculation steps.
Mark scheme (5 marks)
- Assume a 100 g sample so that the percentages can be treated as masses in grams: 75 g of carbon and 25 g of hydrogen
- Divide the mass of carbon by its relative atomic mass: 75 ÷ 12 = 6.25 moles of carbon
- Divide the mass of hydrogen by its relative atomic mass: 25 ÷ 1 = 25 moles of hydrogen
- Divide both molar amounts by the smallest value (6.25) to find the simplest whole number ratio: carbon = 1, hydrogen = 4
- States the empirical formula as CH₄
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Amount of substance questions
- A pharmaceutical company is checking the purity of a newly synthesised compound.…
- A bakery uses sodium hydrogencarbonate (NaHCO₃) as a raising agent in bread. A c…
- A student analyses a compound containing only iron and sulfur. The student finds…
- A chemist is investigating an unknown ionic compound. The compound contains only…