Section-D: Comprehension Type
Vapour pressure of a solvent is the pressure exerted by vapour when they are in equilibrium with its solvent at that temperature. The vapour pressure of solvent is dependent of nature of solvent, temperature, addition of non-volatile solute as well as nature of solute to dissociate or associate. The vapour pressure of a mixture obtained by mixing two volatile liquids is given by PM = where and are vapour pressures of pure components A and B and XA, XB are their mole fractions in mixture. For solute-solvent system, the relation becomes where B is non-volatile solute.
The amount of solute (mol.wt. 60) required to dissolve in 180 g water to reduce the vapour pressure to 4/5 of the pure water :