Define the term iodine number.

Diets that are high in omega-3 fatty acids are recommended as healthy for the heart. Eicosapentaenoic acid \(({M_{\text{r}}} = 302)\) is a common omega-3 fatty acid found in fish oils. Calculate the number of carbon-carbon double bonds in one molecule of this acid if 3.02 g of this acid reacts with 12.7 g of \({{\text{I}}_{\text{2}}}{\text{ }}({M_{\text{r}}} = 254)\).

mass (in g) of I₂/iodine reacting with 100 g of fat/oil/substance/lipid;

Allow “grams of I₂” instead of “mass (in g) of I₂”.

Allow amount/number of mol of I₂ reacting with 1 mol of fat/oil/substance/lipid.

Do not accept mass number for mass.

\(\left( {\frac{{3.02}}{{302}} = } \right){\text{ }}0.0100{\text{ (mol)}}\) acid;

\(\left( {\frac{{12.7}}{{254}} = } \right){\text{ }}0.0500{\text{ (mol) }}{{\text{I}}_2}\);

\(\left( {\frac{{0.0500}}{{0.0100}} = } \right){\text{ }}5\) (carbon-carbon double bonds);

Award [1 max] for 5 if no working shown.

The better candidates gave the correct definition of iodine number, namely the mass in grams of iodine reacting with 100 g of fat. This is the precise definition of iodine number that should be emphasised to candidates in the teaching programme. Some candidates incorrectly stated amount in grams instead of mass in grams of iodine, which showed poor understanding of the inherent difference between mass, measured in grams, and amount, measured in moles. The iodine number calculation on calculating the number of C=C in eicosapentaenoic acid was well answered and full marks were typically scored.

The better candidates gave the correct definition of iodine number, namely the mass in grams of iodine reacting with 100 g of fat. This is the precise definition of iodine number that should be emphasised to candidates in the teaching programme. Some candidates incorrectly stated amount in grams instead of mass in grams of iodine, which showed poor understanding of the inherent difference between mass, measured in grams, and amount, measured in moles. The iodine number calculation on calculating the number of C=C in eicosapentaenoic acid was well answered and full marks were typically scored.