State and explain :
Raoult's law for non-volatile solute.

Explanation

If the solute is non-volatile then relative lowering of vapour pressure is equal to mole fraction of solute which is non-volatile.
`P_(S)=P_(A)+P_(B)`
`P_(S)=P_(A)` (`therefore` Solute B is nonvolatile)
`P_(S)=P_(A)^(@)*chi_(A)`
`P_(S)` = Vapour pressure of solution `chi_(A)` = mole fraction of solvent in solution.
`P_(A)^(@)` = Vapour pressure of the pure solvent.