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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 |
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