what is the relationship between relative lowering of vapour pressure and elevation in boiling point.?
The boiling point elevation can be explained in terms of vapor pressure.
Vapour pressure is a measure of the tendency of the solution molecules to escape to gas phase. A liquid boils when its vapor pressure is equal to the surrounding pressure. The addition of a solute to a solvent dilutes the solution, and decreases the vapor pressure: the molecules escape into the environment more slowly. For the vapor pressure to exceed the external pressure a higher temperature is required; thus, a higher boiling point is observed.
In general, if something has a high vapor pressure, it is expected to have a low boiling point. If it has a low vapor pressure, it is likely to have a high boiling point.