A weighed amount of anhydrous calcium chloride is taken in the set of U-tubes at the end.Īll 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 first three bulbs contain a weighed amount of the solution under examination and the next three bulbs contain a weighed amount of the pure solvent. The complete assembly is shown in the figure. As the solvent is usually water the reagent is generally anhydrous Calcium Chloride. 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. Measurement of Vapour Pressure Lowering (Ostwald and Walker’s Apparatus) The total vapour pressure is given by line marked III in the figure. (It can be seen from the plot that pA and pB are directly proportional to xA and xB respectively).

The dashed lines I and II represent the partial pressure of the components. The relationship between vapours pressure and mole fraction of an ideal solution at constant temperature. If the vapour behaves like an ideal gas, then according to Dalton’s law of partial pressures, the total pressure P is given by Where XA is the mole fraction of the component A given by nA/nA+ nB xB is the mole fraction of the component B, given by nB/nA+nB and are the vapour pressures of pure components A and B respectively. Suppose a binary solution contains nA moles of a volatile liquid A and nB moles of a volatile liquid B, if PA and PB are partial pressure of the two liquid components, the according to Raoult’s law The partial pressure of any volatile component of a solution at any temperature is equal to the vapour pressure of the pure component multiplied by the mole fraction of that component in the solution. Vapour pressures of a number of binary solution of volatile liquids such as benzene and toluene at constant temperature gave the following generalization which is known as the Raoult’s law.

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