Answers and comments

(The minimum answer required on the markscheme for each mark is given in the centre first.)

This question essentially asks if students can recognise three of the functional groups listed in Topic 10 – organic chemistry (alky, alkenyl, hydroxyl, carbony, carboxyls, halogenoalkyl, amine, ester and the phenyl group) when presented in an unfamiliar molecule.

(b)

This is another example of being asked to convert a mass in grams to an amount in moles. Since the mass was given to three significant figures and the molar mass is to at least three significant figures the answers should also be given to three significant figures although the slash in the answer above shows that 2.3 x 10^-2 was acceptable.

The equation gives the information that one mole of aspirin is formed from one mole of salicylic acid. Hence the maximum amount of aspirin that can be formed is 2.28 x 10^-3 mol or 4.11 g.

The percentage yield is simply the mass actually obtained divided by the maximum amount that could be obtained expressed as a percentage.

Since the mass is given as 2.50 g the zero is significant hence the value is quoted to three significant figures.

This is an interesting question as there are several possible answers. Clearly the true yield cannot be more than 100%. Since the calculation was correct it follows that the product must have contained something else apart from pure aspirin. If some of the salicylic acid had not reacted and was contaminating the product then the product would have a mass less than the 100% yield as the M_r of salicylic acid is less than the M_r of aspirin. It is possible that the product is contaminated with some of the excess ethanoic anhydride or some of the ethanoic acid which is also formed in the reaction. However this is very unlikely as both are soluble in water and would have been removed during the recrystallization process. It follows therefore that the most likely explanation is that the aspirin has not been dried properly after the recrystallization process and contains water.

Section A of Paper 3 will always the nature of science and this may well involve the use of an hypothesis. Often this will occur in the data response question. The logic behind the reasoning of the hypothesis is suspect. Just because the C-C bonds are all equal it does not follow that the carbon to oxygen bonds must also all be the same. In general double bonds are shorter than single bonds so one would expect the C to O bonds to be different.

This question tests the definition of an acid in terms of Brønsted -Lowry theory and its conjugate base. Ethanoic acid is a weak acid so its conjugate base, the ethanoate ion will be a reasonable base. Higher Level students should also be able to define an acid in terms of the ionic theory and Lewis theory.