A bakery uses sodium hydrogencarbonate (NaHCO₃) as a raising agent in bread. A chemist analyses the composition of sodium hydrogencarbonate and states that its empirical formula is NaHCO₃. Explain what is meant by the term 'empirical formula' and describe how a chemist would determine the empirical formula of a compound from data showing the mass of each element present in a sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium hydrogencarbonate contains sodium, hydrogen, carbon and oxygen. A 16.8 g sample of sodium hydrogencarbonate contains 6.9 g of sodium, 0.2 g of hydrogen, 2.4 g of carbon and 7.2 g of oxygen.
Model answer (5 marks)
The empirical formula is the simplest whole‑number ratio of atoms of each element in a compound.
To find it from mass data:
1. Divide the mass of each element by its relative atomic mass to obtain the number of moles of that element.
2. Divide each mole value by the smallest mole value obtained.
3. The resulting numbers give the simplest whole‑number ratio of atoms.
4. If the ratios are not whole numbers, multiply all of them by the same integer to obtain whole numbers.
For NaHCO₃ the calculation gives a 1:1:1:3 ratio, so the empirical formula is NaHCO₃.
To find it from mass data:
1. Divide the mass of each element by its relative atomic mass to obtain the number of moles of that element.
2. Divide each mole value by the smallest mole value obtained.
3. The resulting numbers give the simplest whole‑number ratio of atoms.
4. If the ratios are not whole numbers, multiply all of them by the same integer to obtain whole numbers.
For NaHCO₃ the calculation gives a 1:1:1:3 ratio, so the empirical formula is NaHCO₃.
Examiner tips
- Use the exact wording ‘simplest whole‑number ratio’ to match the mark scheme.
- Show the step‑by‑step calculation – mass ÷ atomic mass, then divide by the smallest mole, then adjust to whole numbers.
- Mention that the final ratio must be whole numbers, otherwise multiply by an integer.
Common mistakes
- Failing to divide by the smallest mole value, giving incorrect ratios.
- Not converting the final non‑whole numbers to whole numbers by multiplying with an integer.
- Using the wrong atomic masses or not showing the calculation steps.
Mark scheme (5 marks)
- The empirical formula is the simplest whole number ratio of atoms of each element in a compound
- Divide the mass of each element by its relative atomic mass to find the number of moles of each element
- Divide all mole values by the smallest mole value obtained
- The resulting whole number ratio gives the empirical formula
- If the ratio values are not whole numbers, multiply all values by an appropriate integer to obtain whole numbers
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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