Elevation of boiling point





Elevation of boiling point

It is known that a liquid boils only when its vapour pressure becomes equal to the atmospheric pressure. But when a non-volatile solute is added to the solvent, the vapour pressure is lowered. In order to boil the solution, the temperature has to be raised further so as to make its vapour pressure equal to atmospheric pressure.

The raising of temperature is known as elevation of boiling point (\Delta T_b). It depends upon the number of moles of the added non-volatile solute.

Thus,

\Delta T_b \propto m \\[3mm] \text{or} \hspace{20mm} \delta T_b = K_b \times m

 

Where K_b = molal boiling point elevation constant and ‘m’ = molality of the solute.

Elevation of boiling point

Van’t Hoff correlated K_b with the heat of vaporization, L (in calories per gram of solvent)

K_b = 0.002 T^2 / L_v

 

Where T = boiling point of solvent in Kelvin.

The equation, \Delta T_b = K_b \times m, may be written as:

\delta T_b = \dfrac{1000 \times K_b \times w}{m \times W} \\[3mm] \text{or} \hspace{10mm} m = \dfrac{1000 \times K_b \times w}{\delta T_b \times W}

where ‘w’ is the mass of solute, ‘W’ is the mass of solvent, ‘m’ is molar mass of the solute, K_b  is the molal elevation constant and \Delta T_b is increase in boiling point.



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