Example 1.6

Example . The vapour pressure of pure benzene at a certain temperature is . bar. A non-volatile, non-electrolyte solid weighing . g when added to . g of benzene (molar mass g mol - ). Vapour pressure of the solution, then, is . bar. What is the molar mass of the solid substance? The various quantities known to us are as follows: p = . bar; p = . bar; M = g mol – ; w = . g; w = g Substituting these values in equation ( . ), we get . bar – . bar . bar – . g × g mol × g M Therefore, M = g mol – The vapour pressure of a liquid increases with increase of temperature. It boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. For example, water boils at . K ( ° C) because at this temperature the vapour pressure of water is . bar ( atmosphere). We have also learnt in the last section that vapour pressure of the solvent decreases in the presence of non-volatile solute. Fig. . depicts the variation of vapour pressure of the pure solvent and solution as a function of temperature. For example, the vapour pressure of an aqueous solution of sucrose is less than . bar at . K. In order to make this solution boil, its vapour pressure must be increased to . bar by raising the temperature above the boiling temperature of the pure solvent (water). Thus, the boiling point of a solution is . . Elevation of Boiling Point always higher than that of the boiling point of the pure solvent in which the solution is prepared as shown in Fig. . . Similar to lowering of vapour pressure, the elevation of boiling point also depends on the number of solute molecules rather than their nature. A solution of mol of sucrose in g of water boils at . K at one atmospheric pressure. Let b T be the boiling point of pure solvent and b