Background information

It is an interesting fact that if you buy generic aspirin tablets they are very much cheaper than packets of branded aspirin. The analysis shows that they all usually contain the same amount of aspirin (usually 300 mg per tablet) but the branded tablets contain other ingredients so contain less aspirin in terms of percentage by mass. This practical enables students to determine the percentage of aspirin in various brands of locally available aspirin tablets. They can then work out the cost per gram of aspirin. For best results do not use soluble aspirin tablets as the other ingredients may interfere with the colour change of the indicator.

I’ve put a brief history of aspirin in the student worksheet but it is worth looking at the information provided by the aspirin foundation. This gives both the history and its uses. Interestingly enough it is being trialled on 400 men in South Wales (where I live) to see if it is effective at preventing the onset of Alzheimer’s disease – but I’ve forgotten the details!! Actually you can bring in some good NOS and TOK here as there is considerable evidence that Felix Hoffmann who worked for the Bayer foundation was not in fact the first person to synthesise aspirin but just the first person to patent it so he is usually credited with the discovery.

This experiment is essentially just an acid-base titration and is suitable for SL and HL. It is, of course, also suitable to cover both of the mandatory laboratory components listed in Topic 1.3 Reacting masses and volumes 'Use of the experimental method of titration to calculate the concentration of a solution by reference to a standard solution' and Topic 8.2 Properties of acids & bases. 'Candidates should have experience of acid-base titrations with different indicators'. The reaction of alcohols with carboxylic acids to form esters is covered in Topic 10.2: Functional group chemistry so even students who are not studying Option D Medicinal chemistry should have some knowledge of the chemistry of esters. All students should be able to name the phenyl, carboxyl and ester functional groups present in aspirin. For HL students you might like to mention that another way of analysing aspirin is to heat it with a known amount of excess sodium hydroxide then titrate the left over sodium hydroxide with hydrochloric acid (an example of a ‘back titration’). When heated one mol of aspirin reacts with 2 mol of sodium hydroxide as the ester is hydrolysed. You could ask them to write the equation for this reaction.

As the students pool their results you could use this experiment as an example of the use of a spreadsheet for data processing and put a number 3 in the ICT box on form 4/PSOW. It is certainly worth discussing with them why branded tables are so much more expensive. Is it just to make the shareholders of pharmaceutical companies rich?

I've included questions in the worksheet below which you could give to students to help them calculate their results.