Preparation of Benzoic Acid (OCR A Level Chemistry)

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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

Examiner Tip

Benzoic acid lends itself particularly well to melting point testing as it

    • does not decompose on heating
    • has a melting point in the range 100-200 oC
    • has a sharp and precise melting point

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.