Practical 10- purification Flashcards
Give the reason to dissolve the impure compound in a minimum volume of hot solvent
appropriate solvent should dissolve both compound and impurities when hot. Compound should not dissolve in when cold. Minimum volume is used to obtain saturated solution and to enable crystallization on cooling
Why hot filter solution through filter paper quickly
removes any insoluble impurities and heat will prevent crystals reforming during filtration
why cool the filtered solution by inserting beaker in ice
crystals will reform but soluble impurities will remain in solution form because they are present in small quantities so solution isn’t saturated. Ice will increase the yield of crystals.
why pour filtrate through the Buchner funnel and filter under reduced pressure to separate out crytsals
the water pump connected will reduce the pressure and speed up the filtration
wash the crystals with distilled water
to remove soluble impurities
Describe the Solid and Purity test to obtain a pure sample of aspirin or benzoic acid
- dissolve the impure compound in a minimum volume of hot solvent
- hot filter solution through filter paper quickly
- cool the filtered solution by inserting beaker in ice
- pour filtrate through Buchner funnel and filter under reduced pressure to separate crystals
- wash the crystals with distilled water
- dry the crystals between absorbent paper
How to use melting point apparatus to decide if a sample is pure
a pure sample has a sharp melting point which will be the same as the data book value
an impure sample will melt over a range of several degrees celcius and the melting point will be lower than the quoted data book value
how to determine an accurate melting point
- use a small sample in the capillary tube
- heat slowly, especially when approaching the expected melting point
-record the values from when the sample first starts to melt into a liquid
purifying an organic liquid
place your crude liquid sample in a separating funnel (after distillation)
add equal volumes of saturated NaCl (aq) solution- this ensures all water and polar compounds separate from the sample and dissolve in NaCl
shake and open separating funnel to release pressure build up
let two layers separate out- there will e 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 Na2SO4 to remove any water, sample should go from cloudy to clear
higher density liquid will form the lower layer
example method for purification of aspirin
- weigh out given mass of salicylic acid
- transfer the contents into the pear shaped flask under the glass rod
- at the fume cupboard, add volume of ethanoic anhydride and phosphoric acid
add anti-bumping granules to prevent the formation of large bubbles
reflux
prepare a beaker with deionized water and ice bath
pour the mixture into the beaker
rinse the inside of the flask to ensure all contents are transferred
stir in water bath and wait
filter the sample using the buchner funnel
wash with de-ionised water
pat with filter paper
draw a diagram of a buchner funnel
refer to phone for the image
Choosing a suitable solvent for recrystallisation
the product is soluble in hot solvent
the product is insoluble in the cold solvent
outline the method that the student should use for this recrystallisation
dissolve in the minimum volume of water
hot solvent
allow solution to cool and allow crystals to form
distil the filtrate and collect distillate
suggest two practical reasons why the percentage yield for this reaction may not be 100%
- sample still wet
2, product still in beaker
explain why the student might prefer to use ethanoic anhydride even though it has a slower rate of reaction
violent reaction, HCl released
