QUESTION 141 (5 marks)
Complete combustion of 0.460 g of a compound containing only C, H and O produces 0.880 g CO₂ and 0.540 g H₂O. Its molecular-ion mass is 46 for a singly charged ion. Use atomic masses C = 12.0, H = 1.0 and O = 16.0. The compound has a broad O–H absorption in its infrared spectrum.
(a) Determine the empirical formula from the combustion data. [3 marks]
(b) Determine the molecular formula and the identity of the compound. [2 marks]
Practice marking scheme
Answer
Empirical and molecular formula ; ethanol.
Working
mol. mol. Their masses are 0.240 g and 0.0600 g, leaving 0.160 g O, or 0.0100 mol. The ratio is 2:6:1. The empirical-formula mass is 46.0, matching the molecular ion. The O–H band identifies the alcohol isomer, ethanol, rather than dimethyl ether.
Marking criteria
- Determines carbon amount 0.0200 mol from CO₂. [1 mark]
- Determines hydrogen amount 0.0600 mol from H₂O. [1 mark]
- Uses the remaining oxygen mass to obtain 0.0100 mol and empirical formula C₂H₆O. [1 mark]
- Uses the molecular ion to confirm the molecular formula C₂H₆O. [1 mark]
- Identifies ethanol using the O–H absorption. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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