A geologist discovers a mineral sample and wants to determine the empirical formula of the iron oxide it contains. The mineral is found to contain only iron and oxygen. Explain how the geologist could use the masses of iron and oxygen in the compound to determine its empirical formula.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a sample of the iron oxide is analysed, it is found to contain 2.24 g of iron and 0.96 g of oxygen. The relative atomic masses are: Fe = 56, O = 16.
Model answer (5 marks)
1. Calculate moles of each element:
Fe: 2.24 g ÷ 56 g·mol⁻¹ = 0.04 mol
O: 0.96 g ÷ 16 g·mol⁻¹ = 0.06 mol
2. Divide by the smallest mole value (0.04 mol):
Fe: 0.04 ÷ 0.04 = 1
O: 0.06 ÷ 0.04 = 1.5
3. Convert the ratio to whole numbers by multiplying by 2:
Fe: 1 × 2 = 2
O: 1.5 × 2 = 3
4. Empirical formula: Fe₂O₃ (Fe : O = 2 : 3).
Fe: 2.24 g ÷ 56 g·mol⁻¹ = 0.04 mol
O: 0.96 g ÷ 16 g·mol⁻¹ = 0.06 mol
2. Divide by the smallest mole value (0.04 mol):
Fe: 0.04 ÷ 0.04 = 1
O: 0.06 ÷ 0.04 = 1.5
3. Convert the ratio to whole numbers by multiplying by 2:
Fe: 1 × 2 = 2
O: 1.5 × 2 = 3
4. Empirical formula: Fe₂O₃ (Fe : O = 2 : 3).
Examiner tips
- Show each calculation step and units; use the correct atomic masses.
- State the mole ratio before converting to whole numbers; explain the multiplication factor.
- Write the final empirical formula clearly.
- Mention that the ratio Fe : O = 1 : 1.5 before scaling to 2 : 3.
Common mistakes
- Using the wrong atomic mass for Fe or O.
- Failing to divide by the smallest mole value before scaling.
- Giving the ratio 1 : 1.5 as the final formula instead of 2 : 3.
Mark scheme (5 marks)
- Divide the mass of each element by its relative atomic mass to find the number of moles of each element
- Correctly calculates moles of iron as 0.04 mol (2.24 ÷ 56)
- Correctly calculates moles of oxygen as 0.06 mol (0.96 ÷ 16)
- Divide both values by the smallest number of moles to find the simplest whole number ratio
- States the empirical formula is Fe₂O₃, giving the ratio Fe : O = 1 : 1.5 → 2 : 3
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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