Linoleic acid is an essential fatty acid whose formula is given in Table 22 of the Data Booklet. Determine the mass of iodine, I₂, which reacts with 100 g of linoleic acid.

Fats, such as butter, are solid triglycerides. Explain why fats have a higher energy value than carbohydrates.

The formula of stearic acid is also given in Table 22 of the Data Booklet. Explain why linoleic acid has a lower melting point compared to stearic acid.

2 mol of iodine reacts with 1 mol of linoleic acid;

\({M_{\text{r}}} = {\text{253.80 g}}\,{\text{mo}}{{\text{l}}^{ - 1}}\) for iodine and \({M_{\text{r}}} = {\text{280.50 g}}\,{\text{mo}}{{\text{l}}^{ - 1}}\) for linoleic acid;

\(\left( {\frac{{(507.60 \times 100.00)}}{{280.50}} = } \right){\text{ }}180.96{\text{ (g)}}/181{\text{ (g)}}\);

Award [3] for correct final answer.

Allow 254 g mol^–1 for iodine and 281/280 g\(\,\)mol^–1 for linoleic acid.

Award [2 max] for incorrect ratio calculation to give answers such as 90.4, 90.481, 90.7 (g) depending on M_r values used.

less oxidized (compared to carbohydrates) / fewer oxygen atoms (compared to carbohydrates);

C=C’s in linoleic acid cause the chain to be more uneven/kinked;

linoleic acid cannot pack as closely as stearic acid;

intermolecular/van der Waal’s/London/dispersion forces weaker in linoleic acid;

Accept converse argument for stearic acid.

In (a) the vast majority of candidates did not recognise that linoleic acid had two C=C double bonds and hence the ratio n(I₂):n(acid) = 2:1. Candidates also made careless errors in calculating the molar mass of either/both iodine or/and linoleic acid.

Part (b) was answered well by about half of the candidates.

In Part(c) many candidates scored both marks, but there were cases which indicated poor preparation of candidates on this rather trivial question which appears so often in examinations.