Question: in relative lowering of vapor pressure when a non volatile solute is added to a volatile solvent the vapor pressure decreases .why does the vapor pressure decrease??  Edit

Answer: ok, lets us assume, u have taken a half filled water glass with a lid on it. Now all liquids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their liquid form. Now evaporation only takes place on the surface of the liquid so this vaporization depends on on the surface area of the water in contact with air, that means surface area of our half filled glass.

But when we will add non volatile solute to it, it will dissolved in the water. Previously the upper surface area of water contained only water molecules, but now few of them will be replaced by non volatile solute molecules, so the vapor pressure will be decreased.

The decrease in vapor pressure is governs by Raoult's law.

According to Raoult’s law (1887), at any given temperature the partial vapor pressure (pA) of any component of a solution is equal to its mole fraction (XA) multiplied by the vapor pressure of this component in the pure state (PoA) . That is, PA = PoA x XA,  Edit