Aspirin Synthesis of an essential Medicine

Aspirin Synthesis of an essential Medicine


This lab aimed to become familiar with the synthesis process of a vital medicine Aspirin from the precursors, i.e., salicylic acid, acetic anhydride, and ester formation in an acidic medium. The synthesis process consists of important steps with necessary precautions. An essential step in this experiment was testing the purity of newly synthesized crystals of Aspirin with the FeCl3 test.

Chemicals and Equipment

Salicylic acid Acetic anhydride

Phosphoric acid (Conc.) Acetylsalicylic acid

FeCl3 solution

Erlenmeyer flask 125 mL Graduated cylinders 10mL, 100 ml.

Vented stopper Beaker, 400 mL

Hot plate Heat-resistant gloves

Ice bath Filter paper

Büchner funnel Side arm filter flask

Watch glass Drying oven

Test tubes, small (3) Melting point tubes

Melting point apparatus


  1. 3 g of salicylic acid was weighed and placed into a 125 ml Erlenmeyer flask
  2. 6.0 mL of acetic anhydride was added and swirled in the flask to disperse the solid
  3. Five drops of concentrated phosphoric acid were added and swirled again to mix. Stoppered the flask with one of the vented stoppers supplied
  4. The Erlenmeyer flask was heated in a hot-water bath made of a 400 ml beaker. When water was closed to boiling, 90oC heat-resistant blue gloves were used to supply with occasionally swirling the contents to mix until the reaction was homogeneous.
  5. After 20 minutes, the flask was removed from the hot-water bath, and added 2 mL of water with swirling
  6. The flask was allowed to cool until it could be handled. Then 40 ml of water was added with mixing. Then the flask was put in an ice bath and allowed crystals to grow
  7. The product was filtered through a piece of filter paper in a Büchner funnel using the water aspirator for a gentle vacuum. The product was rinsed with a generous amount of water several times and removed all possible water
  8. The product was transferred from the filter paper to a drying oven to dry it to a constant weight

Testing the purity of the sample

1) Minimal amount of three samples were put into separate test tubes

  • Product
  • The salicylic acid
  • The pure sample of acetylsalicylic acid (Aspirin)

2) 2 mL of water was added to each mixing and a couple drops of the FeCl3 solution. Swirled to mix and recorded the observations, especially the appearance and color of the product compared to the other two samples.


Amount of salicylic acid used = 3 g

Molar mass of salicylic acid = 138 g/mol

Moles of salicylic acid = 3/138

= 0.022 mol

According to the balanced chemical equation

1 mol of salicylic acid gives = 1 mol of Aspirin


0.022 Mol of salicylic acid gives = 0.022 mol of Aspirin


Moles of Aspirin = 0.022 mol

Molar mass of aspirin = 180 g/mol

Mass of Aspirin produced = moles x molar mass

= 0.022 x 180

= 3.96 g

So theoretical yield is = 3.96 g

Actual yield of Aspirin = 3 g

% yield = 3/3.96 x 100

= 75.76%

The melting point of crystals of Aspirin prepared from the synthetic process was 129.1 to 133.5 oC, but the actual melting point of pure Aspirin is 135.1 to 139.9 oC. The difference in melting point arises from the fact that the synthesized crystals of Aspirin contain impurities.

FeCl3 test is used to test the purity of aspirin crystals. Phenols give feCl3 test. When the test is carried out, if the purple-red color is seen, it is impure because it contains salicylic acid.


From the experiment performance, the synthesis process of Aspirin, an ester formation reaction, was studied, and observations were made. A small amount of acid was used as a catalyst which speeds up the reaction.

The observed melting point was less than the pure aspirin crystals due to the impurities present. These impurities, acetic anhydride, salicylic acid, and 4-hydroxybenzoic acid, are major constituents of impure Aspirin. Due to these impurities, the newly synthesized Aspirin cannot be used for headaches.

One of the primary sources of errors results from the exposure of Aspirin produced to the prolonged drying process. Insufficient heating or more heating also affects the amount of Aspirin produced. Crystals may be lost during the filtration process and recrystallization.

The overall experiment performance was exciting and informative as well. Conditions and the effect of each reagent were come to know and how these may affect the actual yield. There was also an essential concept of impurities present, which made our synthesized Aspirin not safe for use.

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