Q76 · Practice questionSimple familiar4 marks
QUESTION 76 (4 marks)
The illustrative IR spectrum belongs to one of these compound classes: alkane, alcohol, ketone or carboxylic acid.
a) Identify the most likely class and cite two diagnostic spectral features. [3 marks]
b) State one limitation of using only this IR spectrum to identify an exact molecule. [1 marks]
WORKED SOLUTION
4 marksPractice marking scheme
Carboxylic acid; a strong C=O absorption near 1700 cm⁻¹ and a very broad O–H band extending through approximately 2500–3300 cm⁻¹. IR does not alone establish the complete carbon skeleton.
The combination of a carbonyl and the exceptionally broad acid O–H distinguishes a carboxylic acid from the other listed classes. Several acids may share these functional groups.
- Identifies carboxylic acid. [1 marks]
- Cites strong carbonyl absorption. [1 marks]
- Cites broad acid O–H absorption. [1 marks]
- Explains functional-group evidence is not a unique whole-molecule identification. [1 marks]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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