Required Practical 10 - Preparation of pure organic solid/liquid

Question 1

Suggest role of sulfuric acid

Answer 1

Catalyst

Question 2

Explain why it is important filter paper is wet during filtration of organic solid

Answer 2

• filtration proceeds faster

- prevents clogging of filter by solid

Question 3

Suggest why a glass rod is used when transferring solutions

Answer 3

• prevent splashing

- control flow

Question 4

Suggest why it is important not to remove funnel while water pump is still on

Answer 4

Water pump creates a vacuum which could damage filter paper if funnel is removed resulting in loss of filtered solid

Question 5

Suggest why crude aspirin in added to ethanol AND cold water during purification

Answer 5

• ethanol added to dissolve everything
• cold water added to precipitate out aspirin ONLY
• ethanol alone causes aspirin to remain in solution
• cold water alone would precipitate out impurities

Question 6

Suggest why prolonged heating of aspirin during purification should be avoided

Answer 6

To ensure aspirin does not decompose

Question 7

Suggest why mixture is cooled in an ice bath during purification of aspirin

Answer 7

Aspirin more likely to precipitate out giving a better yield

Question 8

Give reasons for loss in product during preparation and purification of solid

Answer 8

• reaction may not go to completion (reversible)
• side reactions may take reactants away
• not all substance is recrystallized
• lost in filtering/ transferring liquids

Question 9

Suggest how the purity of an organic solid can be determined

Answer 9

• chromatography should only show one spot
• pure solid will melt at a precise temperature
• impure solid will melt over a WIDE range of temperatures + BEFORE the actual melting point

Question 10

Suggest what is meant by glacial acid

Answer 10

Pure acid (not in solution)

Question 11

Suggest why sodium carbonate is added in purification of organic liquid

Answer 11

Removes traces of carboxylic/sulphuric acid in distillate

Question 12

Suggest why stop cock is opened in between inversions of separating funnel in preparation of organic liquid

Answer 12

• neutralisation of sodium carbonate produces CO2

- opening tap allows release of pressure which could blow stopper out of funnel

Question 13

Suggest why remaining organic liquid from separating funnel is poured in a dry boiling tube containing anhydrous sodium sulfate

Answer 13

Dehydrating agent which removes traces of water

Question 14

Suggest why a minimum volume of a hot solvent is used when purifying an organic solid

Answer 14

• obtain a saturated solution

- enable crystallisation upon cooling

Question 15

Suggest safety precautions taken in preparation of pure organic solid

Answer 15

gloves - reactants are corrosive
water bath - avoids overheating
anti-bumping granules - slows down boiling

Question 16

Suggest how to decide which solvent to use when purifying an organic solid

Answer 16

• dissolve when hot to remove insoluble impurities

- precipitate out when cold to remove soluble impurities

Question 17

Suggest why filtration takes place through a pre-heated funnel in preparation of organic solid

Answer 17

So that the solution stays hot and does not recrystallize

Question 18

Suggest organic solid is washed with a little cold solvent

Answer 18

To wash off any other soluble impurities

Question 19

Suggest reason for drying organic solid

Answer 19

Remove any remaining solvent

Question 20

Suggest why organic solid is packed densely into capillary tube when testing purity

Answer 20

You need enough solid to see it melt clearly

Question 21

Suggest why it is important to ensure the apparatus heats up very slowly as the expected melting point of organic sold approaches

Answer 21

So you can observe the exact temperature at which it starts to melt

Question 22

Suggest why concentrated sulfuric acid is added slowly and with cooling

Answer 22

• gives out a lot of heat

- avoid splashing if mixture get hot

Question 23

Suggest advantages of reduced pressure (vacuum)filtration

Answer 23

• quick

- produces relatively dry product

Question 24

Describe preparation and purification of organic liquid

Answer 24

• mix alcohol and carboxylic acid is round bottom flask
• add slowly with cooling a few drops of sulfuric acid
• shake well and reflux
• rearrange apparatus and distill
• transfer to separating funnel
• add sodium carbonate
• invert and open tap
• allow two layers to separate + run off lower layer

Question 25

Describe preparation and purification of organic solid

Answer 25

• mix salicylic acid with ethanoic anhydride / H2SO4
• round bottom flask partially submerged in beaker (water bath)
• reflux
• dissolve in hot solvent sparingly (saturated)
• heated filter (remove insoluble impurities)
• place in cold water ice bath (recrystallize)
• filter under reduced pressure and wash with distilled water (remove soluble impurities)
• allow to dry

Question 26

Suggest why a pure sample of aspirin may appear to melt at a different to temperature to its true melting point

Answer 26

temperature on thermometer is not same as sample

Question 27

State observations that would indicate a sample of aspirin was not pure when measuring its melting point

Answer 27

• melts BEFORE true melting point for aspirin

- melts over a WIDE range of temperature (not sharp)

Question 28

Why is flask cooled before solid crystals are filtered off

Answer 28

yield would be lower if warm