Lowering in vapour pressure is determined by Ostwald and Walker dynamic methed. It is based on the prinicipal , that when air is allowed to pass through a solvent or solution, it takes up solventvapour with it to get itself saturated at that temperature
I and II are weighted separately before and after passing dry air. Loss in mass of each set, gives the lowing of vapour pressure. The temperature of air, the solution and the solvent is kept constant.
Dry air was passed thorough 9.24 g of solute in 108 g of water and then through pure water. The loss in mass of solution was 3.2 g and that of pure water 0.08 g . The molecular mass (g/mol) of solute is nearly :
