Question

How is the molal elevation constant related to the latent heat of vaporisation of the solvent?

Answer 1

The molal elevation constant K_b for a particular solvent can be calculated from the following relationship.

`K_b = (M R T_b^2)/(1000 xx Delta H_v)`    ...(i)

Where,

T_b = Boiling point of solvent

ΔH_v = Molar enthalpy of vaporisation of solvent (Latent heat of vaporisation for one mole of the solvent)

M = Molecular mass of solvent 

R = Gas constant

The latent heat of vaporisation per gram of the solvent (L_v) is related to ΔH_v as

`L_v = (Delta H_v)/M`

Therefore, Eq. (i) can also be written as

`K_b = (R T_b^2)/(1000 L_v)`