Question

Aspirin was synthesised by acetylation of salicylic acid.

(a)Describe briefly how you would recrystallize the crude aspirin.

(b)Assume the crude aspirin was contaminated to the extent of 10% by weight with salicylic acid. The crude material (3.0 g) was dissolved in boiling water (100 mL) and allowed to cool to 20 °C where the solubility of aspirin is 3.0 g L'1 and that of salicylic acid is 2.0 g L'1. Are the crystals that formed pure aspirin? If not, what is the percentage purity by weight?

Answer 1

(a) Recrystallisation is a procedure for purifying compounds. The process of recrystallisation involves dissolution of the solid in an appropriate solvent (or Solvent mixture) at an elevated temperature and the subsequent re-formation of the crystals upon cooling, so that any impurities remain in the solution.

The crude aspirin can be purified by recrystallization. In this purification method, the crude aspirin will be dissolved in a small amount of warm ethanol. Water will then be added and the solution will be cooled slowly and then chilled. The acetylsalicylic acid (asprin) will recrystallize, and the solid impurities (unreacted salicylic acid) should remain dissolved in the solution. The solid aspirin will again be collected using vacuum filtration and tested for purity. This aspirin will be more pure than the crude aspirin.

(b)