Analysis of the content in Aspirin Tablet

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Yu Wing Yee 6A(30)

  1. Title: 

V3 Analysis of the content in Aspirin Tablet

  1. Aim/Objective:

To find out the mass of aspirin in each tablet by using back titration

  1. Theory

In this experiment, hydrolysis and back titration are carried out to determine the mass of the active ingredient aspirin in each aspirin tablet to see if it is the same as that stated on the package. Hydrolysis is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. Since aspirin (2-ethanoylbenzoic acid) can be readily hydrolysed using sodium hydroxide, forming the sodium salts of two weak acids, ethanoic acid and 2-hydroxybenzoic acid, and a water molecule, excess sodium hydroxide is used to react with aspirin in a conical flask initially, as illustrated in the following series of equations:

CH3COOC6H4COOH(aq) + NaOH(aq)CH3COOC6H4COO -Na+(aq) + H2O(l)

CH3COOC6H4COO -Na+ (aq) + H2O(l)  HOC6H4COO-Na+ (aq) + CH3COOH(aq)

CH3COOH(aq) + NaOH(aq) → CH3COO-Na+(aq) + H2O(l)

Overall reaction:

CH3COOC6H4COOH(aq) + 2NaOH(aq)HOC6H4COO-Na+(aq) + CH3COO-Na+(aq) +H2O(l)

The hydrolysis is performed on a Bunsen flame to speed up the reaction. The kinetic energy of molecules becomes greater with an increase in temperature, colliding with each other faster. After warming for at least 10 minutes, all the 2-ethanoylbenzoic acid in the conical flask has reacted with sodium hydroxide, then the remaining sodium hydroxide can be determined using a titration set-up with Sulphuric acid in the burette, with the following equation:

H2SO4(aq.) + 2NaOH(aq.)  Na2SO4(aq) + 2H2O(l)

Below is the titration curve of sodium hydroxide and Sulphuric acid:

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Phenol red, having a pH transitional range of 6.8 to 8.4 is used, because the final pH of the reaction lies between its range.

Diagram of the titration

This kind of titration is known as back titration, as the excess reactant is titrated in order to find out the number of moles of sodium hydroxide used in hydrolysis. Using the equation no. of moles = molarity * volume used (dm3), the number of moles of Sulphuric acid used can be found out and so does that of sodium hydroxide. As the number of moles of sodium ...

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