Question 1: Separation of mixtures
- Extraction
This is a method of separation of two immiscible solvent phases. It is the best method for the separation of a mixture of Cyclohexanone and Acetone (miscible)
Reason:
Cyclohexanone is insoluble in water while Acetone is miscible with water. A mixture of the two compounds forms two distinct layers in water. Distillation cannot be used because the two compounds are highly flammable.
Procedure:
In this method of separation, water is added to the mixture. Acetone completely mixes with water while cyclohexanone does not. A funnel is then used to separate the two.
(Note: the colouration is used to show the difference in the liquid layers. This is not the actual colours).
2. Column chromatography
This is a method of separation of mixtures based on differential adsorption of substance by the adsorbent.
In this case, it is the best method for separation of tetrahydrofuran and acetonitrile.
Reason:
Tetrahydrofuran has low viscosity at standard temperatures while acetonitrile provides a mobile phase in the chromatography. Acetonitrile is also highly flammable and therefore, distillation cannot be used to separate it.
3. Distillation
This is the method of separation used for two liquids with different boiling points, or a solid and a liquid.
It is the best method for separation of benzoic acid and toluene.
Reason:
Benzoic acid is a crystalline solid that results from partial oxidation of toluene.
Question 2: Purity and melting Points
2 a) The melting points are given below. Purity also decreases downwards.
Compound melting point (0C) actual melting point (0C)
Salicylic acid 87-89 87.33
p-dibromobenzene 155-159 159
Benzoic acid 110-122C, 122
2 b) Reasons for variations in melting points
- Impurities – presence of impurities will always lower the melting point.
- Temperature variations.
Question 3: Thin Layer Chromatography
3 a) Column preparation by TLC
A thin layer chromatography of the mixture is run and a sample of a known compound in the solution spotted. The solvent is then allowed to evaporate and the compound re-dissolved in a slightly solvent onto a silica gel TLC plate.
A line is then drawn, about one-half centimeter from the bottom of the plate and the sample spotted on the line.
The spot is kept as small as possible.
The TLC plate is then placed in a developing jar containing an appropriate level of the proper solvent, and the top back of the jar rested.
The solvent is then allowed to travel up the plate to about half a centimeter from the top and the solvent front marked. The plate is then allowed to dry and the spot visualized under UV light.
Finally, the spot concentration is determined.
