How is raoult's law used in determining the molecular weight of a non-volatile solute?

Answer 1

Ostwald and Walker method: The apparatus used is shown in Fig. It consists of two sets

of bulbs. The first set of three bulbs is filled with solution to half of their capacity and second

set of another three bulbs is filled with the pure solvent. Each set is separately weighed

accurately. Both sets are connected to each other and then with the accurately weighed set of

guard tubes filled with anhydrous calcium chloride or some other dehydrating agents like

P2O5, conc. H2SO4 etc. The bulbs of solution and pure solvent are kept in a thermostat maintained at a constant temperature.

A current of pure dry air is bubbled through. The air gets saturated with the vapours in each set of

bulbs. The air takes up an amount of vapours proportional to the vapour pressure of the solution

first and then it takes up more amount of vapours from the solvent which is proportional to the

difference in the vapour pressure of the solvent and the vapour pressure of solution, i.e. p0 - ps. The

two sets of bulbs are weighed again. The guard tubes are also weighed.

Loss in mass(x) in the solution bulbs ∝ ps

Loss in mass(y) in the solvent bulbs ∝ (p0 - ps)

Total loss(x+y) in both sets of bulb ∝ [ps +(p0 - ps)]

Answer 2

For an ideal solution of volatile liquids, the vapour pressure of any component is directly proportional to the mole fraction present in the solution