Required Practical 10
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Describe a method for purifying a sample of aspirin (6 pts)
1) Dissolve the crude aspirin in a minimum volume of hot solvent 2) Filter hot solution through fluted filter paper quickly 3) Slowly cool in an ice bath until the aspirin recrystallises 4) Suction filtrate with a Buchner flask to separate out crystals 5) Wash the crystals with distilled water 6) Dry the crystals with absorbent paper
Write the word equation for the preparation of aspirin
salicylic acid + ethanoic anhydride –> aspirin + ethanoic acid
Why is a minimum amount of hot solvent used in recrystallisation?
To ensure that as much product as possible crystallises out when the solution cools
How are insoluble impurities removed during recrystallisation?
By hot filtration through filter paper
Why is the solution filtered while hot during recrystallisation?
To prevent the product from crystallising in the filter paper
What happens after the solution is filtered?
The filtrate is allowed to cool slowly to room temperature to form crystals of the pure product
Why should the mixture be cooled slowly during recrystallisation?
Slow cooling allows large, pure crystals to form rather than small impure ones
How are soluble impurities removed?
They remain dissolved in the solvent when the pure crystals form and are removed during filtration
After crystals form, what is the next purification steps?
The crystals are filtered under reduced pressure using a Buchner funnel
Label a diagram of Buchner funnel
What is the purpose of washing the crystals with a small amount of cold solvent?
To remove any soluble impurities still on the crystal surface
How is the product dried after filtration?
It’s left in a warm place, in an oven at a suitable temperature.
How is the purity of the organic solid tested?
By measuring its melting point and comparing it to the literature (known) value
What does a pure compound’s melting point look like?
It has a sharp melting range, typically within 1–2°C of the literature value
What does an impure compound’s melting point look like?
It melts over a wide range and at a lower temperature than the pure compound.
Why does impurity lower the melting point of a solid?
Impurities disrupt the regular lattice structure, weakening intermolecular forces
What are key safety considerations in this practical?
• Wear goggles • Handle hot apparatus carefully • Avoid inhaling fumes from organic solvents
How can percentage yield be improved in this practical?
Use minimum solvent Cool slowly Ensure complete drying of product before weighing
Why is yield often less than expected?
• Product is loss during filtration or washing • Incomplete reactions can occur