Empirical Formula Determination
Elemental analysis of an organic compound shows 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass; what is the empirical formula of this compound (C = 12, H = 1, O = 16)?
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About This Question
- Subject
- chemistry
- Chapter
- purification and characterisation of organic compounds
- Topic
- empirical formula determination
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
CH2O
The empirical formula gives the simplest whole-number ratio of atoms in a compound and is found from percentage composition by converting each mass percentage into moles and then dividing by the smallest mole value. For carbon: 40.0 / 12 = 3.33 mol. For hydrogen: 6.7 / 1 = 6.7 mol. For oxygen: 53.3 / 16 = 3.33 mol. Dividing each by the smallest value (3.33) gives C : H : O = 1 : 2.01 : 1, which rounds to 1 : 2 : 1, so the empirical formula is CH2O. The option C2H4O2 is a multiple of the empirical formula and represents a molecular formula, not the simplest ratio, so it is not the empirical formula. The option CHO gives a hydrogen-to-carbon ratio of one, which contradicts the calculated ratio of two. The option C2H6O does not match the computed atom ratios at all and corresponds to a different composition such as ethanol. This mole-ratio procedure is exactly the NCERT method for empirical formulae. Plausibility check: the empirical formula must use the smallest integers, and CH2O satisfies the 1 : 2 : 1 ratio, confirming the answer.
This medium difficulty chemistry question is from the chapter purification and characterisation of organic compounds, covering the topic of empirical formula determination. It appeared in the 2025 exam.
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