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2023 O Level Chem P1 Q37 — Empirical Formula / Alcohols Solution

Empirical Formula / Alcohols 2023 O Level Pure Chemistry · Paper 1 · Q37

Question

An alcohol is completely combusted in excess oxygen. Two products are formed, 1.65 g of carbon dioxide and 0.90 g of water. What is the formula of the alcohol?

ACH₃OH
BC₂H₅OH
CC₃H₇OH
DC₃H₆OH

Correct answer: C

Video Solution

Explanation

Answer: C. Find moles of C and H in the combustion products.

\(n(\text{CO}_2) = 1.65 / 44 = 0.0375\) mol → \(n(\text{C}) = 0.0375\) mol.
\(n(\text{H}_2\text{O}) = 0.90 / 18 = 0.0500\) mol → \(n(\text{H}) = 2 \times 0.0500 = 0.100\) mol.

Ratio C : H = 0.0375 : 0.100 = 3 : 8.

For a monohydric alcohol \(\text{C}_n\text{H}_{2n+1}\text{OH}\): with 3 carbons and 8 hydrogens, n = 3 → \(\text{C}_3\text{H}_7\text{OH}\). Check: 3 C, 7 + 1 = 8 H. ✓ This matches propan-1-ol or propan-2-ol — option C.

Options A (\(\text{CH}_3\text{OH}\)) and B (\(\text{C}_2\text{H}_5\text{OH}\)) have the wrong C:H ratio. Option D (\(\text{C}_3\text{H}_6\text{OH}\)) is not a real alcohol formula — the H count doesn't satisfy \(\text{C}_n\text{H}_{2n+1}\text{OH}\).

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