pratical 9 investiagting ph and pratical 10 preparation of a pure organic solid and a pure organic liquid

Question 1

describe how you would determine an accurate value for the melting point of aspirin (2marks)

Answer 1

heat melting point tube in an oil bath
slowly near the melting point

Question 2

describe the method for recrystallisation

Answer 2

dissolve sample in a minmium volume of hot water
the solution is filtered hot
the filitrate is cooled in ice to form crystals
the crystals are collected by filtration, washed with cold water and left to dry

Question 3

describe why sample is dissolved in a minimum volume of hot water

Answer 3

to obtain saturated solution
to increase yeild
enable crystallisation

Question 4

describe why sample is filtered hot

Answer 4

to remove insoluble impurities

Question 5

why is the sample cooled in ice

Answer 5

increase amount of crystals that are formed

Question 6

why is the sample washed with cold water

Answer 6

to remove soluble impurities

Question 7

give 2 industrial advantages of using ethanoic anhydride rather then ethanoyl chloride in the maufacture of aspirin

Answer 7

ethanoic anhydride is cheaper
less corrosive
reaction is less violent and safer to use

Question 8

describe the method to piurify benzoic acid

Answer 8

Dissolve crude product in hot solvent/water
of minimum volume
Filter (hot to remove insoluble impurities)
Cool to recrystallise
Filter under reduced pressure / with Buchner funnel
wash (with cold solvent) and dry

Question 9

how to test purity of benzoic acid

Answer 9

use mp apparatus heat gently a pure sample has a sharp mp

Question 10

purifying an organic liquid
Cyclohexanol is converted into cyclohexene by dehydration using distillation with concentrated H2SO4.

Answer 10

Once the reaction is complete:
Place your crude liquid sample in a separating funnel. (Often after distillation)
Add an equal volume of saturated NaCl (aq) solution – This ensures all water and polar compounds separate from the sample and dissolve in NaCl solution.
Shake and open separating funnel to release pressure build up.
Let two layers separate out. – There will be an aqueous polar layer and a non-polar organic layer.
Use the tap at the bottom of the separating funnel to run off lower layer.
To complete the purification process add a drying agent to the sample eg anhydrous Na2SO4 to remove any remaining water. Shake and allow to stand. The sample should go from cloudy to clear.