Synthesis of Aspirin.
1) Tare a 250 mL conical flask and into it place approximately 3.00 grams of salicylic acid . Record the mass of salicylic acid to the nearest 0.01 gram.

2) Take the flask containing the salicylic acid to the a fume cupboard and add 10 mL of acetic anhydride and 1 mL of concentrated sulfuric acid. Swirl and heat on a hot plate at at 50oC until the salicylic acid has dissolved.

 
3) Take the flask off the hot plate and allow to cool. Then add 100 mL of near freezing, cold water kept in an ice bath. Small crystals of the acetylsalicylic acid (aspirin) should begin to appear as the mixture cools. Crystallisation can be increased by scratching the internal surface of the flask with a glass rod while swirling the contents. Tee water in the flask should start to look cloudy as the precipitation takes place.
After a while the crystals should grow in size and the water becomes clear, as shown on the right.

4) Set up the Vacuum Filtration apparatus.

 
5) Weigh a filter paper
6) Filter the acetylsalicylic acid from the funnel. Rinse the precipitate with distilled water.
Step 7 Use a small spatula to remove the filter paper and aspirin. Allow the precipitate to dry overnight before weighing.
Step 8 Weigh the precipitate and filter paper.

From the results shown above
1. Determine the mol of salicylic acid used if the molar mass of aspirin is 180.16 g/mol and the molar mass of salicylic acid is 138.12 g/mol.
Solution
2. Calculate the mass of acetic anhydride from its volume (10 mL) and density (1.08 g/mL)
Solution
3. Calculate the mol of acetic anhydride
Solution

4. On the left is the chemical equation for the formation of aspirin.
i. Which reactant is in excess?
Solution

ii. What is substance A?
Solution

5. Calculate the theoretical yield of aspirin
Solution

6. Calculate the percent (%) yield from the actual product mass .
Solution

i. Is this percentage yield possible? Explain.

ii. Discuss a possible error that would cause this yield.

7. FeCl3 turns from a yellow to a purple colour in the presence of salicylic acid, as shown on the right. How can iron(III)chloride be used to check the purity of aspirin formed?

Now do your own synthesis and determine the percentage yield from your own results.