Q78 · Practice questionComplex familiar5 marks
QUESTION 78 (5 marks)
A compound contains 54.5% C, 9.1% H and 36.4% O by mass. Its molecular-ion peak is at (singly charged). The IR spectrum has a strong carbonyl band and a broad acid O–H band. Use C = 12.0, H = 1.0, O = 16.0.
a) Determine the molecular formula. [3 marks]
b) Give two non-cyclic carboxylic-acid structural isomers consistent with the evidence. [2 marks]
WORKED SOLUTION
5 marksPractice marking scheme
; butanoic acid and 2-methylpropanoic acid.
For 100 g, moles are 4.54 C, 9.10 H and 2.275 O, giving C₂H₄O (44 g mol⁻¹). Molecular mass 88 doubles this. Both four-carbon acid isomers fit the IR groups.
- Converts composition to mole ratios. [1 marks]
- Obtains empirical formula C₂H₄O. [1 marks]
- Uses molecular ion to obtain C₄H₈O₂. [1 marks]
- Gives butanoic acid. [1 marks]
- Gives 2-methylpropanoic acid. [1 marks]
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