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Investigatory Project on Depression in Freezing Point

 

INVESTIGATORY PROJECT ON STUDY OF THE DEPRESSION IN FREEZING POINT OF A SOLVENT ON THE ADDITION OF A NON-VOLATILE SOLUTE

 

INTRODUCTION

 

Freezing point of a substance is the temperature at which the solid and the liquid forms of the substance are in equilibrium and have the same vapour pressure. It is observed that when a non-volatile solute is added to a solvent, the freezing point of the solvent is lowered. Depression in freezing point is a direct consequence of the lowering of vapour pressure.

 

Depression of freezing point is a colligative property. It is given by the relation

ΔTf = 1000 Kf wB/MBwA

where

ΔTf  = depression in freezing point

Kf = molal depression constant or cryoscopic constant

wB = weight of solute

MB = molecular mass of the solute and

wA = weight of solvent

 

Freezing point of camphor is 179°C is and the molal depression constant for it is 37.7 K.kg.mol-1. Quite a number of organic substances such as biphenyl, acetanilide etc dissolve in camphor giving solutions.

 

AIM OF THE PROJECT

 

To study the depression in freezing point of camphor (solvent) on the addition of biphenyl (solute).

 

APPARATUS AND CHEMICALS

 

1. Thiele’s melting point apparatus

2. Capillary tubes

3. Thermometer

4. Spot plate

5. Chemical balance

6. Liquid paraffin

7. Naphthalene

8. Biphenyl

 

PROCEDURE

 

Determine the melting point of pure camphor using Thiele's apparatus. For this, camphor is powdered using a spatula and porcelain tile. A fine capillary tube is taken and one end of it is fused by showing it to the flame. The finely powdered substance is introduced into the capitally tube to get a column of the substance of length of about 0.5cm. About three fourth of Thiele's tube is filled with liquid paraffin. The bulb of the thermometer is wetted with liquid paraffin and the capitally tube is carefully allowed to cling to the thermometer in such a way that the bulb of the thermometer and the specimen are as close as possible. The apparatus is now closed with the cork in such a way that the thermometer is introduced into the liquid. The open end of the capillary tube must be above the surface of liquid paraffin. The apparatus is clamped on an iron stand and heated slowly by waving the flame at the bend of the side tube. Due to the movement of heated liquid molecules, the liquid paraffin will be uniformly heated up. During heating the mercury thread slowly rises and at a particular point, the substance in the capillary tube suddenly shrinks and melts. The temperature at which shrinking occurs is recorded as the melting point of the substance. The melting point of a solid substance is the same as the freezing point of the liquid substance.

 

Weigh about 3.0 g of camphor and 0.1 g of biphenyl and mix them together in a dry test tube. The tube is corked and heated carefully. When the solid melts and form a homogenous solution, it is poured into one of the depressions in a spot plate. Allow it to cool and solidify. When it is solidified, it is powdered and its melting point is determined using Thiele's apparatus. Record the melting point which is equivalent to the freezing of the liquid.

 

Now mix 3.0 g of camphor with 0.2 g of biphenyl in another dry test tube. Heat it exactly as in the above case to form a homogenous solution and allow it to solidify by pouring into another depression of the spot plate. Determine the melting point of this solid using Thiele's apparatus. Repeat the experiment with 3.0 g of camphor and 0.3 g of biphenyl. Record the observations.

 

OBSERVATIONS

 

Melting point of pure camphor =

 

No:

Mass of camphor (solvent) WA

Mass of biphenyl (solute) WB

Molality of soln (m)

m = (WB x 1000)/ (MB x WA)

Melting point of solid obtained by cooling liquid mixture (°C)

Depression in melting point (ΔTf)

1

 

 

 

 

 

2

 

 

 

 

 

3

 

 

 

 

 

 

CONCLUSION

1. Addition of biphenyl (non volatile solute) to camphor lowers the freezing point of camphor.

2. Depression in freezing point depends on the quanity of non volatile solute.

3. Depression of freezing point is found to be proportional to the molality of the solution.

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