We know that, the relative lowering of vapour pressure of a solution is equal to the mole-fraction of the solute present in the solution
P0−pS
P0
=x1 P0= Vapour pressure of solvent PS= Vapour pressure of solution x1= Mole-fraction of solute Thus, option a is true. but the option (b), (c) and (d) are wrong. (b) Because movement of solvent molecules from the lower concentration side to higher concentration side through semipermeable membrane is known as osmosis. (c) According to elevation of b.p., the boiling point of solution is always higher than the solvent. (d) The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to atmospheric pressure (760mm)