= Solution
\b[Two <molecules> of the <carboxylic acid> form a cyclic carboxylic-acid <dimer>.] Let Ar denote the meta-methylphenyl group. One <molecule> donates its OH <hydrogen> to the other's <carbonyl> <oxygen>, and the second donates back:
The two contacts close an eight-membered hydrogen-bonded ring, shown explicitly below.
Both <molecules> therefore act once as hydrogen-bond donor and once as hydrogen-bond acceptor, giving two O–H···O=C contacts. This is the conventional self-complementary <dimer> motif; the schematic is not a prediction of every weaker contact in an unprovided <crystal structure>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2021/paper-1-b5-homodimers.png]
{title=Original hydrogen-bonding structures for the two pure-compound homodimers}
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