According to Raoult’s law,
Psolution xA , where xA = mole fraction of solvent.
Psolution = k . xA , where k = proportionality constant
For a pure solvent,
Vapour pressure = P°, xA = 1
P°solution = k x 1 = k
Substituting P°solvent in Raoult’s law
Psolution = P°solvent . xA
Relative lowenng of vapour pressure
substituting Psolution as P°xB in the above eaquation Relative lowering of vapour pressure
xA + xB = I
where xB = mole fraction of solute. It is clear that the relative lowering of vapour pressure depends only on the mole fraction ofthe solute (xB) and is independent of its nature. Therefore relative lowering of vapour pressure is a colligative property.