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1.4.2 Raoult’s According to Raoult’s law, the vapour pressure of a volatile component 0 Law as a in a given solution is given by p i x i .iIn the solution of a gas in a special case liquid, one of the components is so volatile that it exists as a gas and of Henry’s we have already seen that its solubility is given by Henry’s law which states that Law p = KHx. If we compare the equations for Raoult’s law and Henry’s law, it can be seen that the partial pressure of the volatile component or gas is directly proportional to its mole fraction in solution. Only the 0 proportionality constant K Hiffers from p 1 . Thus, Raoult’s law b0comes a special case of Henry’s law in which K beHomes equal to p 1. 1.4.3 Vapour Another important class of solutions consists of solids dissolved in Pressure of liquid, for example, sodium chloride, glucose, urea and cane sugar in Solutions of water and iodine and sulphur dissolved in carbon disulphide. Some Solids in physical properties of these solutions are quite different from those of Liquids pure solvents. For example, vapour pressure. Liquids at a given temperature vapourise and under equilibrium conditions the pressure exerted by the vapours of the liquid over the liquid phase is called vapour pressure [Fig. 1.4 (a)]. In a pure liquid the entire surface is occupied by the molecules of the liquid. If a non-volatile solute is added to a solvent to give a solution [Fig. 1.4.(b)], the vapour pressure of the solution is solely from the solvent alone. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature. In Fig. 1.4: Decrease in the vapour pressure of the the solution, the surface has both solute and solvent on account of the presence of solvent molecules; thereby the fraction of the solute in the solvent (a) evaporation of the surface covered by the solvent molecules gets molecules of the solvent from its surfacereduced. Consequently, the number of is denoted by , (b) in a solution, solutsolvent molecules escaping from the surface particles have been denoted byand they also occupy part of the surface area. is correspondingly reduced, thus, the vapour pressure is also reduced. The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature. For example, decrease in the vapour pressure of water by adding 1.0 mol of sucrose to one kg of water is nearly similar to that produced by adding 1.0 mol of urea to the same quantity of water at the same temperature. Raoult’s law in its general form can be stated as, for any solution the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction. In a binary solution, let us denote the solvent by 1 and solute by 2. When the solute is non-volatile, only the solvent molecules are present in vapour phase and contribute to vapour pressure. Let p be 1 Chemistry 12 Reprint 2026-27