Page 18 - Chemistry - XI

EXPERIMENT - 7

Objective

To obtain pure crystals of benzoic acid from its impure sample.

Materials Required
Impure benzoic acid, fi lter paper, benzene, and china dish

Procedure

1. Take benzene (20 mL) in a beaker and add impure sample of benzoic acid. Dissolve it completely.
2. Filter the solution to remove insoluble suspended impurity.
3. Keep the china dish in air for some time so that most of the benzene gets vapourised and crystallisation
point is reached.
4. Crystals of pure benzoic acid will be obtained.

5. Decant off the mother liquor and dry the sample in the folds of the fi lter paper.

Observations
Colour of the crystals : Colourless
Shape of the crystals : _____________

Yield (amount of crystals formed) : _____________

Precautions
1. The vapours of benzene should be out of contact of fl ame.
2. If heating is required, it should be done on water bath only.

VIVA VOCE

Q1. What is solubility?
Ans. It is defi ned as the maximum amount of solute which can be dissolved in 100 g of solvent.

Q2. What is the formula of potash alum? Is it a double salt or complex salt?
Ans. K SO .Al (SO ) .24H O. It is a double salt.
4
4 3
2
2
2
Q3. What is the formula of copper sulphate?
Ans. CuSO .5H O
4 2
Q4. What is the formula of benzoic acid?
Ans. C H COOH
5
6
Q5. What is meant by crystallisation?
Ans. When a hot saturated solution is cooled, well defi ned geometrical shaped crystals are formed. This
process is called crystallisation.

Q6. What do you mean by a saturated solution?
Ans. Solution in which no more amount of solute can be dissolved is called a saturated solution.

Q7. What do you understand by mother liquor?
Q7. What do you understand by mother liquor?
Ans. The solution which is left behind after separating the crystals from saturated solution is called
Ans. The solution which is left behind after separating the crystals from saturated solution is called
mother liquor.
mother liquor.
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