Safety Precautions
Sodium hydroxide and sulphuric acid solutions are corrosive and irritant.
Wearing safety goggles and lab coats during the heating stage of the experiment
Results
Weighting
Brand name: Halewool Chemicals Ltd.
No. of tablets:3
Mass of aspirin tablets= 1.0181 g
Observations
When the aspirin was heated with sodium hydroxide, the aspirin tablets dissolved and only some white powder was left in the flask.
When the diluted reaction mixture was titrated with the standard sulphuric(VI) acid , the colour of mixture changed from pink to pale yellow.
Table
Mean vol. of standard sulphuric(VI) acid =14.40
Concentration of standard sulphuric(VI) acid=0.0503M
Calculation
2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
No. of mol of H2SO4 used to neutralize NaOH(aq)= 0.0144 x 0.0503
= 7.2432 x 10-4 mol
No. of mol of NaOH used to neutralize H2SO4(aq) = 7.24329 x 10-4 x 2
= 1.44864 x 10-3 mol
No. of mole of NaOH(aq) used in hydrolysis of 2-ethanoloxybenzoinc acid
=1 x 0.025 – 1.44864 x 10-3 x 10
=0.0105136mol
No. of mole of 2-ethanoloxybenzoinc acid in the weighed sample =0.0105136/2
=0.0052568 mol
Mass of 2-ethanoloxybenzoinc acid in 3 Aspirin Tablets =0.0052568 x 180
=0.946224g
Mass of 2-ethanoloxybenzoinc acid in 1 Aspirin Tablet = 0.946224/3
= 0.315408g
Price of 1 tablet = 30/100
= 0.3$
Price per g of 2-ethanoloxybenzoinc acid = 0.3/0.315408
= 0.951$/g
Discussion
In the hydrolysis of the aspirin, the 25cm3 of distilled water added was not necessarily be accurate, because the number of moles of aspirin and sodium hydroxide was still the same in the conical flask no matter how much distilled water was added to it. Also, the distilled water should be in excess when compared to aspirin. This can ensure all the acid in aspirin was hydrolyzed and even some distilled water was left for evaporation during boiling. After all, the reaction mixture in the conical flask would be transferred into a volumetric flask with more distilled water for dilution, so the amount of distilled water added was not that important to be accurate.
Moreover, after the mixture in the conical flask was warmed, it ought to be cooled down, since the volume of the solution is different at different temperature. If the contents in the conical flask were immediately poured into a volumetric flask after warming and distilled water was then filled up to the graduation mark, after some time of cooling, the volume of the solution would increase, meaning too much distilled water was added. The solution would become more diluted, which implies that less number of moles of sodium hydroxide would be pipetted each time. The calculated number of moles of sodium hydroxide reacted with aspirin would be larger. Eventually the calculated mass of aspirin in each tablet would be heavier.
Errors and improvements
In this experiment, there were some errors which might lead to a slightly wring answer.
Firstly, it was assumed that only aspirin in the tablets would react with sodium hydroxide and that other contents in the tablets would not be hydrolyzed. However, if there were such contents which had reacted with sodium hydroxide, a greater amount of sodium hydroxide would be used for hydrolysis, leaving less sodium hydroxide in the conical flask afterwards. As a result, less sulphuric acid would be used to neutralize sodium hydroxide in titration, giving a greater calculated mass of aspirin than actual.
On the other hand, since Sulphuric acid is quite hygroscopic, it would absorb water vapour from the air, diluting itself. If it is left to stand in air for too long, the volume of Sulphuric acid in the burette would become slightly larger and more diluted. So the number of moles of Sulphuric acid per cm3 would decrease and a slightly greater volume of Sulphuric acid would be required to neutralize sodium hydroxide. Then the calculated number of moles of sodium hydroxide used in hydrolysis would be smaller, and that the calculated mass of aspirin would be smaller. To eliminate this error, not only should the titration be carried out faster, the Sulphuric acid should also be kept in air as short as it could.
There are human errors when using the pipette or burette to measure the volume of solution, because the reading was not accurately obtained. In order to minimize the error, we should take the reading at our eye level, and from the bottom of the meniscus.
Besides,when the reaction mixture of tablets and NaOH was boiling, some might be split out. The data would be affected and caused the inaccuracy. Therefore, we should regulate the distance between the bunsen burner and the flask to prevent the misture boiled too vigorously.
Moreover, if the burette was not correctly rinsed of distilled water and sulphuric acid, the acid would be contaminated and reduce our accuracy of calculating.In order to minimize the error, we should rinse the burette with distilled water and then the sulphuric acid solution a few times.
Conclusion
The results obtained from titrimetric analysis of different aspirin show the price of 1g of 2-ethanoloxybenzoinc acid in different brands of aspirin,is shown as below,
As the price of 1g of 2-ethanoloxybenzoinc acid in Halewool Chemicals Ltd. is the cheapest and Cartia is the most expensive, Halewool Chemicals Ltd. aspirin tablets are the most worth to buy and the Cartia’s is the least worth to buy.
