Calculate the iodine number for linolenic acid, ${{\text{C}}_{{\text{17}}}}{{\text{H}}_{{\text{29}}}}{\text{COOH (}}{{\text{M}}_{\text{r}}} = 278.48{\text{)}}$. The condensed structural formula of linolenic acid is given in table 22 of the data booklet.

${{\text{n}}_{{\text{iodine}}}} = {\text{3}}{{\text{n}}_{{\text{linolenic acid}}}}$;

${\text{iodine number }}\left( { = \frac{{3 \times 100 \times 253.8}}{{278.48}}} \right) = 273{\text{(g)}}$;

Award [2] for final correct answer.

If alternative definition, in terms of moles/C=C bonds is given, award [1 max] for iodine number = 3.

Quite a few candidates were aware of the benefits of linolenic acid, though identifying the number of carbon-carbon double bonds present, and using this to calculate the iodine number, was rather more challenging.