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8.5 Boiling Point and Freezing Point of Solution

The temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure is termed as the boiling point.

The boiling point of any liquid increases by adding a non-volatile solute to it. This increases the vapor pressure.

Elevation in the boiling point ( D Tb ) is directly proportional to the amount of solute present.

Concentration of the solute is expressed in molalities (m)

Thus D Tb = mKb

m = concentration in molalities

Kb = molal boiling point elevation constant.

The temperature at which the solid and liquid are in equilibrium is termed as the freezing point.


The freezing point of any liquid decreases by adding solute to it.

Depression in the freezing point ( D Tf ) is directly proportional to the amount of solute present.

Thus D Tf = mKf

m = molal concentration

Kf = molal freezing point depression constant

The following table lists molal boiling point elevation constants and molal freezing point depression constants of a few solvents.

Solvent

Boiling Point 0C

Kb
0/mol

Freezing Point 0C

Kf
0/mol

Acetic acid

Water

Benzene

Ethanol

CCl4

Chloroform

118.1

100.0

80.1

78.4

76.8

61.2

3.070

0.512

2.530

1.220

5.020

3.630

16.6

0.0

5.5

-114.6

- 22.8

- 63.5

3.90

1.86

5.12

1.99

29.80

4.68

Index

8.1 Nature of Solution
8.2 Heat of Solutions
8.3 Concentration of Solution
8.4 Effect of Temperature and Pressure on Solubility
8.5 Boiling Point and Freezing Point of Solution

Chapter 9





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