CH3COOC6H4COOH + 2NaOH → CH3COONa + HOC6H4COONa + H2O
The unused sodium hydroxide which remains is then titrated with standard acid. The amount of alkaline required for the hydrolysis can now be calculated and from the above equation, the number of moles of acetylsalicylic acid which have been hydrolyzed can be found.
Requirements:
Aspirin tablets (about 3), 25cm³ pipette, pipette filler
Burette and stand, small beaker
250 cm³ volumetric flask, 250 cm³ conical flask
Filter funnel, Bunsen burner
Tripod and gauze
Procedure:
Part 1 : Standardize the approximately 1M NaOH used for the hydrolysis
1. Using a safety filler and pipette, exactly 25cm³ of the approximately 1M NaOH solution was transferred into a 250 cm³ standard flask and made up to the mark.
2. 25 cm³ of this solution was titrated against 0.0962M hydrochloric acid using phenolphthalein indicator.
Part 2 : Hydrolyse the aspirin
1. 3 aspirin tablets were weighted into a clean conical flask.
2. Using a safety filler and pipette, 25cm³ of the approximately 1M NaOH was transferred on the tablets, followed by about the same volume of distilled water.
3. The mixture was heated gently for 10 minutes to hydrolyse the acetyl-salicylic acid. 4. 4. Safety glasses were worn.
5 The mixture was cooled and transferred with washings to a 250cm³ standard flask and was made up to the mark with distilled water.
Part 3 : Estimate the quantity of unused NaOH after the hydrolysis
1. 25cm³ of the hydrolysed solution was transferred by a pipette into a conical flask.
2. The solution was titrated against 0.0962M hydrochloric acid using phenolphthalein indicator.
Data/Observation:
Part 1:
Titration number Trial 1 2
Initial burette reading/ cm³ 0.40 14.90 0.20
Final burette reading/ cm³ 25.10 39.30 24.60
Titre/ cm³ 24.70 24.40 24.40
Mean Titre/ cm³ (24.40+24.40)/2 = 24.40
Part 3:
Titration number Trial 1 2
Initial burette reading/ cm³ 0.70 0.30 5.50
Final burette reading/ cm³ 7.70 7.00 12.20
Titre/ cm³ 7.00 6.70 6.70
Mean Titre/ cm³ (6.70+6.70)/2 = 6.70
1. Molarity of standard hydrochloric acid = 0.0962M
2. Mass of 3 aspirin tablets = 1.76g
3. The color of indicator changed from red to pink in both titration.
4. The aspirin tablets were white in color.
5. The aspirin tablets dissolved in sodium hydroxide solution to give a colorless solution.
6. Bubbles were formed while heating.
7. No choking or pungent smell was detected.
8. The experiment was carried out at room temperature.
Calculation:
Part 1:
1. No. of moles of diluted 25cm³ of sodium hydroxide
= 0.0962 x 24.40/1000 = 0.00235 mol
2. No. of moles of diluted 250cm³ of sodium hydroxide
= 0.00235 x 10 = 0.0235 mol
3. Molarity of approximately 1M sodium hydroxide
= 0.0235/0.025 = 0.939 M
Part 3:
1. Initial no. of moles of sodium hydroxide
= 0.939 x 25/1000 = 0.0235 mol
2. No. of moles of sodium hydroxide unused
= 0.0962 x 6.7/1000 x 10 = 0.00645 mol
3. No. of moles of sodium hydroxide used
= 0.0235 – 0.00645 = 0.0171 mol
4. No. of moles of 2-ethanoyloxybenzoic acid
= 0.0171/2 = 0.00855 mol
5. Mass of 2-ethanoyloxybenzoic acid
= 0.00855 x (12x9+8+16x4) = 1.54g
6. Percentage of 2-ethanoyloxybenzoic acid in aspirin tablets
= 1.54/1.76x100% = 87.5%
Conclusion:
The percentage of 2-ethanoyloxybenzoic acid in aspirin tablets is 87.5%.
Questions:
1. Why is the mixture simmered gently and carefully during hydrolysis?
This can increase the rate of reaction.
2. Why is it unwise to boil it vigorously?
The solution which is hot and contain corrosive sodium hydroxide will spill out if we
boil it vigorously.
3. What might the remainder of the tablets be made of?
The tablets might be made of some acidic impurities.
Source of errors:
1. The aspirin tablets may contain more than one kind of acid, titration of the aspirin will neutralize not only the 2-ethanoyloxybenzoic acid.
2. The aspirin tablets may not fully dissolve in the distilled water and sodium hydroxide solution.
3. The mixture of aspirin solution may not be well-shaken.
4. Extra drops of hydrochloric acid may added while the end-point is reached since the end-point is difficult to detect.
5. The space between the tip of burette and the tap may not filled with solution before titration.
6. Some solution may stick on the tip of burette.
7. It was not easy to get a good estimation to 0.05 cm³ from the burette.
8. Some solution may evaporate and escape while heating.
Precaution:
1. Hydrochloric acid and sodium hydroxide is corrosive. We should avoid contacting the solution with our skin.
2. Safety goggles must be worn during heating and titration.
3. The solution should be heated gently to prevent it from spilling out.
4. We should wash our hands after the experiment.