practical 10

• why is the minimum volume of hot solvent used?

  to ensure the hot solution is saturated / crystals would form on cooling (maximise yield)
• why was the conical flask left to cool before the crystals were filtered off?

  the yield would be lower if warm / solubility is higher if warm
• why were the crystals compressed in the funnel?

  so that air passes through the crystals not just round it
• why was a little cold solvent poured through the crystals?

  to remove any soluble impurities
• Describe a method for purifying a sample of aspirin

  1. filter from the reaction mixture
  2. dissolve in the minimum volume of hot solvent
  3. filter through a hot funnel
  4. cool the mixture to allow recrystallisation
  5. filter under reduced pressure through a Buchner funnel
  6. wash with cold solvent
  7. allow to dry
• why is the solution filtered hot?

  removes insoluble impurities and prevents crystals from forming during filtration (maximise yield)
• describe a method to purify an organic liquid

  1. put the impure distillate in a seperating funnel
  2. wash product using sodium hydrogencarbonate solution or saturated sodium chloride solution
  3. allow layers in funnel to seperate, then run and discard the aqueous layer
  4. run the organic layer into a dry, clean flask
  5. add a drying agent
  6. decant the liquid into the distillation flask and distill to collect the pure product
• how can sodium hydrogen carbonate solution be used to wash the impure product?

  removes acidic impurities by neutralisation - shake and release pressure from carbon dioxide produced.
• how can saturated sodium chloride solution be used to wash the impure product?

  helps to separate the organic and aqueous layers
• what drying agent should be used to dry the organic liquid?

  anhydrous sodium sulphate - drying agent should be insoluble and not react with the organic liquid
• how can melting point determination be used to test the purity of an organic product?

  melting point can be measured using an electronic melting point machine or by attaching a capillary tube to a thermometer immersed in heating oil, it is heated up slowly to the melting point

  recorded melting point can be compared to quoted value in data source
• how may the melting point of an impure substance vary from that of a pure substance?

  melting point will be lowered and the sample will melt over a range of several degrees