Required practical 10a

Question 1

What is the aim

Answer 1

To make aspirin
Also learn how to do recrystallisation to purify product

Question 2

How is aspirin made

Answer 2

• 6g of salicylic acid to a weighing boat
• Record mass on scales, then tip the mass into 100cm3 conical flask and reweigh the boat
• Add 10cm3 ethanoic anhydride and swirl it, then add 5 drops of concentrated sulfuric acid
• Warm in a 60 degrees water bath
• Then allow the flask to cool and add contents to 75cm3 of cold water in a beaker
• Aspirin will start crystallising out
• Use buchner filtration under reduced pressure to filter it out

Question 3

How do we do recrystallisation of aspirin to make it more pure

Answer 3

• Add the smallest amount possible of hot ethanol so it dissolves both the aspirin and its impurities
• You want to use the minimum amount of solvent so that the crystals can reform easily
• Let cool so the crystals form
• Then pour over 40cm3 cold water
• Do buchner filtration under reduced pressure
• Blot the product with filter paper to remove excess water

Question 4

What effect do impurities have on melting point of a solid

Answer 4

They lower melting point

They also increase the range of melting point, so a pure substance has a sharp and sudden change from solid to liquid whereas impurities increase this range in melting.

Question 5

How does melting point of product indicate purity of it

Answer 5

The closer the melting point is to data book value, the more pure the substance is

Question 6

Why might you add some cold ethanol to it

Answer 6

To form bigger crystals

Question 7

What would you expect the difference in appearance be between pure aspirin and crude aspirin

Answer 7

Pure is whiter/lighter in colour

Pure aspirin has bigger crystals

Question 8

Why is the minimum quantity of hot solvent added

Answer 8

To allow the crystals to reform when cooled

Question 9

Why was the flask left to cool before crystals were filtered off

Answer 9

In hot solvent, they are more soluble so the yield will be much higher if solution was cooled

Question 10

Sometimes a cork is used to compress the crystals in the funnel.

Answer 10

This is to pass air through the crystals so they dry better

Question 11

Why is cold water poured through the crystals

Answer 11

To dissolve soluble impurities so they’re washed away