Question

Vapour pressure of a solvent containing nonvolatile solute is ___?
A.More than the vapour pressure of the solvent.
B.Less than the vapour pressure of solvent.
C.Equal to the vapour pressure of solvent.
D.None.

Answer 1

Hint: The vapour pressure of any liquid is defined as the pressure exerted by the particles of the liquid that escape its surface and transform into the gaseous phase. The rate at which the liquid vaporizes depends on the surface area exposed to the atmosphere.

Complete answer:
A liquid is said to be boiling only when the vapour pressure of the liquid becomes equal to the external (in most cases the atmospheric) pressure.
A liquid in its pure state has a fixed escaping tendency, entropy and a surface area exposed to the atmosphere at a particular temperature. The addition of a non-volatile solute to a solvent affects its entropy, reduces its escaping tendency and lowers its vapour pressure.
A liquid solution that has low vapour pressure requires a higher amount of thermal energy to allow its particles to escape as gaseous molecules and as a consequence the boiling point of the solution is increased as compared to the boiling point of liquid in its pure state.
Therefore, the vapour pressure of a liquid decreases upon the addition of a non-volatile solute and option (B) is correct.

Note:
A non-volatile solute refers to a solid solute or a liquid solute that does not have enough vapour pressure of its own. Such a solute would only occupy the surface available for solvent molecules to vaporize and escape but will not add up any vapour pressure to the existing vapour pressure of the solvent.