Measurement of Lowering of Vapour Pressure by Ostwald and Walker Method - QS Study

Measurement of Lowering of Vapour Pressure by Ostwald and Walker Method

A number of other devices have been developed for measurement of difference of vapour pressure. All these are known as static methods to differentiate them from the dynamic method due to Ostwald and Walker which is described below.

Figure: Transpiration method of Ostwald and Walker

In this method a stream of dry air is bubbled successively through (i) the solution (ii) the pure solvent and (iii) a reagent which can absorb the vapour of the solvent. As the solvent is usually water the reagent is generally anhydrous Calcium Chloride.

In this method the solution and the solvent are taken in a series of gas washing bottles (Figure) and a slow stream of dry air or nitrogen is bubbled through the solution and the solvent. The stream of gas coming out through D is saturated with the solvent vapour which is absorbed in a series of absorption tubes containing a suitable absorbing agent. The dry gas after passage through the solution becomes saturated with the solvent vapour from the pure solvent because the vapour pressure of the solvent is higher than that of the solution. The gas on passage through the solvent carries with it more vapour from it. After a sufficient flow of the gas the absorption tubes and the two series of washing bottles are separately weighted. The increase in the mass of the absorption tube is proportional to the vapour pressure of the solvent and the loss in mass of the bottles containing solution is proportional to the vapour pressure of the solution, which loss in mass of the bottles, B, is proportioned to the lowering of vapour pressure. Thus,

(p0 – p) / P0 = [loss in mass of B / loss in mass of C]

Since the concentration of the solution is known the molecular mass of the solute is easily calculated. If temperature control is good and this experiment is carried out carefully, the method is capable of giving results of high accuracy. The method is simple and inexpensive. If water is used as the solvent, the absorption tube may be filled with fused anhydrous calcium chloride. The last tube should be guarded against moisture absorption from the atmosphere. If organic solvents are used, as often is the case, the absorption tubes are dispensed with and the results can be obtained by weighing the tubes, A and B. The current of gas should be passed slowly otherwise fine stream of liquid drops may escape with the gas. The total volume of the gas passed should be large so that appreciable loss in mass in the two bulbs may occur.