Preparation and purification of organic compoundsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student has prepared crude aspirin and purifies it by recrystallisation, using a hot mixture of ethanol and water as the solvent. Aspirin is much more soluble in the hot solvent than in the cold solvent.

   (a)

   Why is the minimum volume of hot solvent used to dissolve the crude aspirin?

   [1 mark]

   • ASo that the aspirin reacts completely with the solvent
   • BSo that the impurities dissolve and the aspirin does not
   • CSo that the solution is saturated and most of the aspirin crystallises on cooling
   • DSo that the solvent boils at a lower temperature

   (b)

   How does the melting behaviour of impure aspirin compare with that of pure aspirin?

   [1 mark]

   • AIt melts at a higher temperature over a narrow range
   • BIt melts at the same temperature over a narrow range
   • CIt melts at a higher temperature over a wider range
   • DIt melts at a lower temperature over a wider range

   (c)

   After filtering the crystals, they are washed with a small volume of ice-cold solvent. Explain why the solvent is cold and why only a small volume is used.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student prepares ethyl ethanoate by heating ethanol and ethanoic acid with a little concentrated sulfuric acid, then distils the mixture. The distillate contains ethyl ethanoate, ethanoic acid, ethanol and water. It is shaken with aqueous sodium carbonate in a separating funnel. The density of ethyl ethanoate is 0.90 g cm⁻³ and that of the aqueous layer is about 1.0 g cm⁻³.

   (a)

   What is the main purpose of washing with aqueous sodium carbonate?

   [1 mark]

   • ATo remove the acidic impurities, mainly ethanoic acid
   • BTo react with the ethanol and form a salt
   • CTo remove water from the ester
   • DTo hydrolyse the ester back to ethanoic acid

   (b)

   Which statement about the layers in the separating funnel is correct?

   [1 mark]

   • AThe organic layer is at the bottom and is run off first
   • BThe organic layer is at the top, so the lower aqueous layer is run off first
   • CThe two liquids form one layer
   • DThe organic layer is at the bottom because the ester is denser than water

   (c)

   The stopper is removed and the tap opened from time to time while the funnel is shaken. Explain why, including an equation.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A liquid Y is immiscible with water and boils at 190 °C, but it starts to decompose above about 150 °C. It is present in a reaction mixture together with a large amount of a black, non-volatile tar. A chemist uses steam distillation to separate Y from the tar.

   (a)

   Explain why steam distillation is a suitable method for separating Y from the tar.

   [3 marks]

   (b)

   The distillate is a cloudy mixture of Y and water. Describe how a pure, dry sample of Y can be obtained from it.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student prepares aspirin by warming 2.00 g of salicylic acid with an excess of ethanoic anhydride and a few drops of concentrated phosphoric(V) acid in a water bath. The student then isolates and purifies the solid. The dry, recrystallised product has a mass of 1.80 g and melts over the range 128–133 °C. The literature melting temperature of pure aspirin is 136 °C. Relative formula masses: salicylic acid 138.0; aspirin 180.0.

   (a)

   Describe how the student should isolate the crude aspirin from the reaction mixture and then purify it, giving a reason for each key step.

   [6 marks]

   (b)

   Calculate the percentage yield of aspirin, then use the melting range to evaluate the purity of the product and suggest two reasons why the yield is less than 100%.

   [6 marks]

   Total for question 4: 12 marks

End of questions