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Methods of separating and purifying substances

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123
Question 3

During the purification of a crude sample of benzoic acid (melting point 122∘C122^\circ\text{C}122∘C) via recrystallisation, a student performs the following sequential steps:

  1. Dissolve the impure solid in the minimum volume of hot water.
  2. Filter the hot solution through a heated funnel to remove insoluble impurities.
  3. Allow the filtrate to cool slowly to room temperature, then place it in an ice bath.
  4. Filter the purified crystals under reduced pressure using a Büchner funnel.
  5. Wash the crystals.
  6. Dry the crystals in a desiccator.

Which of the following experimental errors or modifications would result in a decreased yield of pure benzoic acid, without directly decreasing the purity of the collected crystals?

Washing the crystals in Step 5 with a large volume of hot water instead of a small volume of ice-cold water.

Skipping Step 2 (hot filtration) when insoluble sand is present in the crude sample.

Plunging the hot filtrate directly into an ice bath in Step 3, skipping the slow cooling stage.

Failing to dry the crystals fully in Step 6 before weighing them.

Methods of separating and purifying substances Questions

  1. GCSE
  2. /Chemistry
  3. /Methods of separating and purifying substances