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2021 O Level Chem P1 Q34 — Organic Chemistry / Formulae Solution

Organic Chemistry / Formulae 2021 O Level Pure Chemistry · Paper 1 · Q34

Question

Which statements about the formulae of some organic compounds are correct? 1 The general formula of an alkane is CₙH₂ₙ₊₂. 2 The empirical formula of butene is CH₂. 3 The molecular formula of propanol is C₃H₇O.

A1 and 2 only
B1 and 3 only
C2 and 3 only
D1, 2 and 3

Correct answer: A

Video Solution

Explanation

Answer: A (1 and 2 only). Statement 1 is true: the general formula of an alkane is CₙH₂ₙ₊₂. Statement 2 is true: butene is C₄H₈, so the ratio C:H is 4:8 = 1:2, giving the empirical formula CH₂.

Statement 3 is false: the molecular formula of propanol is C₃H₈O (propanol is C₃H₇OH), not C₃H₇O as written. So statement 3 is incorrect.

Only statements 1 and 2 are correct. The trap is statement 3: read the formula carefully — C₃H₇O is missing a hydrogen and is not a valid molecule, whereas the correct molecular formula of propanol is C₃H₈O.

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