Experiment: Analysing molar mass of Aspirin


In 1899 the Bayer Company of Germany introduced the ethanoate ester of salicylic acid naming it, ‘Aspirin’. Aspirin is an acid, it can be titrated with a base such as sodium hydroxide to the equivalence point. The following chemical equation describe the acid-base reaction that will be observed in this experiment.

H-Aspirin + NaOH ——> Na-Aspirin + H2O

(acid) (base) (salt)

The above chemical equation is balanced as written. An indicator will be used to provide a way to visually determine the endpoint of the titration.

Phenolphthalein changes from colourless to a faint pink while bromothymol blue changes from yellow to a faint blue at a pH of around 8.


To calculate the molar mass of Aspirin


The chemical formula of Aspirin is C9H8O4. It can also be written as C6H5(OCOCH3)COOH. In this experiment, Aspirin, which is an acid, will react with sodium hydroxide, NaOH in an acid-base reaction to produce salt and water. This analysis makes use the fact that aspirin is a monoprotic acid and therefore reacts with NaOH according to the equation:


The experimental molar mass of Aspirin, C6H5(OCOCH3)COOH is close to the theoretical value.

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The difference between the experimental value and the accepted value is not really big. However, there are difference between the experimental molar mass of aspirin when using phenolphthalein and bromothymol blue as indicator. The obtained molar mass of aspirin when using phenolphthalein indicator is smaller than using bromothymol blue as indicator. The difference with the theoretical value is also less.

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Due to this difference, the percentage error when using phenolphthalein is smaller than bromothymol blue. Therefore, phenolphthalein is a better indicator in this acid-base titration reaction. This is because the final pH or the endpoint of titration lies in between pH range of phenolphthalein that is 8.2-10.0 and not in bromothymol blue which is 6.0-7.6 . This is the reason why phenolphthalein indicator is able to produce more accurate result than bromothymol blue.


There are some weaknesses that were detected throughout the experiment. Firstly, the volume of indicator are not fix in this experiment. Using drops is not really suitable as it may distort the volume of indicator in this experiment. The difference may not really big but it still can affect the final result.

Secondly, the mixture of aspirin, C6H5(OCOCH3)COOH and sodium hydroxide, NaOH is not stirred during the titration. This may produce uneven and not uniform result as the NaOH is not totally mix with the aspirin. The neutralization process may only occur on the surface of the aspirin solution.

Having said that, this experiment is successfully analyzing the molar mass of aspirin. The experimental value is close to the theoretical value.

Further improvement

In order to improve the results of the future experiments, several ways can be implemented. Firstly, the volume of indicator use should be fixed instead of using drops. By using this way, the level of uncertainty of the final results can be reduced. Besides that, the mixture of aspirin solution and NaOH need to be stirred throughout the titration process. More uniform results can be obtained by implementing this method.

Cite this page

Experiment: Analysing molar mass of Aspirin. (2017, Nov 11). Retrieved from https://studymoose.com/experiment-analysing-molar-mass-of-aspirin-essay

Experiment: Analysing molar mass of Aspirin

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