Preparation of Benzoic Acid (OCR A Level Chemistry A): Revision Note

PAG 6.2: Preparation of Benzoic Acid

• Benzoic acid is a suitable substance for purification by recrystallisation as it

  • Readily available in most school labs

  • Dissolves in hot water, but not in cold water

  • Is relatively safe to handle in the laboratory

Key steps in the procedure

• 1.0 g of impure benzoic acid is added to a small conical flask and the flask is placed on a hot plate

• Using a supply of hot water, the minimum amount of hot water is added to the flask to dissolve the benzoic acid

  • It should be around 30-35 cm³ of water

• When is it dissolved a hot filtration is carried out to remove any insoluble impurities

• To do this a pre-heated glass funnel is rested on a hot conical flask and a piece of fluted filter paper is added

• The funnel should have a short stem to avoid the product solidifying prematurely and blocking the funnel

• The filtrate is allowed to cool and crystallise

  • Sometimes crystallisation can be difficult to achieve, so agitating the flask or scratch the inner walls with a glass rod can help initiate the process

• The solid product is then recovered by Buchner filtration, washed and dried