Recrystallisation
What I am going to do is:
- Pour the mixture through a buchner funnel.
- Apply suction using a filter pump and wash the crystals with ice cold water.
- Dry the crystals and carefully weigh them
- Transfer the crystals into a beaker and add 8cm3 ethanol stir the mixture and warm gently to dissolve the crystals.
- When they have dissolved pour 15 cm3 of warm water into the alcohol.
- Cover the beaker and wait till the cools and crystals start to form.
- Place beaker into ice to complete crystallisation.
- Pour the contents into a Buchner funnel and apply suction.
- Dry the crystals and re-weigh them. Calculate the percentage yield
- Check the melting point.
Health and safety
Ethanoic anhydride is corrosive and flammable. Phosphoric acid is corrosive so be careful and when using harmful substances do in a fume cupboard.
Problems, which may occur
The problems, which may occur, are:
- When refluxing the mixture the water bath may not be hot enough and the reaction may not take place. To overcome this check the temperature before if it is not quite high enough the leave the substance in for more than 5 minutes.
- When weighing the product at the end make sure you do not lean on the table as this will change the weight and the percentage yield will not be as accurate.
- Some of the substances we are using may be harmful so handle them with care.
- Impurities to over come this make sure all the apparatus are washed with deionised water to make sure there are no contaminants.
- Apparatus error. Make sure the balance is accurate before weighing out the substances.
- When drying the crystals the weight of the product can be affected because if it is not dried properly then the yield will be higher and it will not be pure aspirin.
Apparatus
The apparatus, which I used, were:
- Hydroxybenzoic acid
- Pear shaped flask
- Ethanoic anhydride
- Phosphoric acid
- Boiling water
- Condenser
- Water bath
- buchner funnel
- Balance
- Beaker
Risk assessment
Results
Appearance= white crystals
Weight = before crystallisation = 2.860 weight after crystallisation =1.972g
Melting point= 134°c
By looking at the results, I can see that before crystallisation the substance was not pure as it weighed more.
`Calculating the percentage yield
Results
Salicylic acid acetylsalicylic acid
C7 H6 O3 C9 H8 04
+
Mr of Salicylic acid = C7 H6 O3
12x7 + 1x6 + 16x3=138
Mr of acetylsalicylic acid = C9 H8 04
12x9 + 1x8 + 16x4=180
We used 2g of, salicylic acid.
N = m
n= moles m= mass mr = relative molecular mass
mr
2
= 0.014.
138
We started of with 0.014 moles of salicylic acid .
The amount of acetylsalicylic acid we should have made is:
M= n x r
n= moles
m= mass
mr = relative molecular mass
M= 0.014 x 180 = 2.52g
The weight of acetylsalicylic acid should have been 2.52g
The weight of acetylsalicylic acid obtained was 1.972g
Percentage yield = actual yield
X 100 =78 %
Expected
Conclusion
The percentage yield I got was lower than expected I think this was because we may have lost some when:
- Filtering the substance as it may have been stuck to the filter paper.
- When we used the funnel maybe not all of the substance went through
- When drying the substance some of it may have been stuck to the paper towel.
The melting point I got was the same as the expected melting point; this shows that the substance was pure.
