Lowering in vapour pressure is determined by Ostwald and Walker dynamic method. It is based on the principle, that when air is allowed to pass through a solvent or solution, it takes up solvent vapour 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 lowering of vapour pressure. The temperature of air, the solution and the solvent is kept constant.