Aspirin has a chemical formula of 2-acetoxybenzoic acid (CH3COOC6H4COOH) and it is synthesized from salicylic acid or 2-hydroxybenzoic acid. If the synthesis reaction is incomplete, the unreacted salicylic acid is present as a contaminant and we have a less than 100% yield of aspirin. There are a number of methods to determine the purity or % yield of aspirin.
One of the methods is the titration of a given aspirin sample in a 50/50 mix of ethanol and water, against sodium hydroxide. The number of moles of sodium hydroxide used to neutralize the acid can be used to determine the molar mass of a given aspirin sample. It would be somewhere between the molar mass of aspirin (180 g) and that of salicylic acid (138 g), and this will give a measure of the % purity of the given aspirin sample.
A spectrophotometer can also be used to determine the purity, since aspirin makes a violet complex with iron and this can be measured by using a spectrophotometer. Different concentrations of aspirin can be used to draw a calibration curve and a given sample can be checked against this curve.
Hope this helps.
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