Let p0 = Vapour pressure of pure solvent
p = Vapour pressure of solution
Pressure at level L1 = p0; Pressure at level L2 = p. Pressure at L1 will be greater than pressure at L2. Then
P0 – p = hD …(ii)
where ‘d’ is density of solvent vapour of pressure p0.
Dividing Eq. (i) by (ii), we have,

at a fixed temperature d/D is constant.
? p = constant (p0 - p) …(iii)
i.e., p ? (p0 – p) or
p …(iv)
Thus, osmotic pressure is directly proportional to the lowering of vapour pressure.
Regards
Arun (askIITians forum expert)