Measurement of Relative Lowering of Vapour Pressure using Ostwald and Walker's Apparatus

Ostwald and Walker apparatus is the arrangement to calculate relative lowering in vapour pressure. In this method, a stream of dry air is bubbled successively through (i) the solution (ii) the solvent and (iii) a reagent which can absorb the vapours of the solvent. Generally solvent is water and reagent is anhydrous CaCl₂.

The first three bulbs contain a weighed amount of the solution and the next three bulbs contain a weighed amount of the pure solvent. A weighed amount of anhydrous CaCl₂ is taken in the U tube. All the bulbs must be kept at the same temperature and air must be bubbled gradually to ensure that it gets saturated with the vapours in each bulb.

The dry air, as it passes through the solution, takes up an amount of vapour which is proportional to the vapour pressure of the solution at the prevailing temperature. This moist air passes through water (solvent), it takes up a further amount of vapour which is proportional to the difference in vapour pressure of pure solvent and the solution.

i.e., loss in mass of solution	vapour pressure of solution
	P
loss in mass of solvent	vapour pressure of pure solvent - vapour pressure of solution
	Po - P

and
 

Loss in mass of solution + loss in mass of solvent  P + P0 - P  P0

This air is then passed through CaCl₂ of U-tube.

The CaCl2 tubes are weighed at the end of the experiment. The gain in mass should be equal to the total mass of the solution and solvent which is proportional to P_o.

i.e.