Question

Which of the following is correct regarding vapour pressure of the solution formed when a non-volatile solute (solid) is added in a pure volatile solvent?

• A. Vapour pressure of the solution increases.
• B. Vapour pressure of the solution decreases.
• C. Vapour pressure of the solution remains the same.
• D. Vapour pressure of the solution may increase or decrease.

Answer 1

The correct option is B Vapour pressure of the solution decreases.

Relative Lowering of Vapour Pressure (RLVP)​:
V.P. of a solution containing a non-volatile solute (solid solute) is always found to be less than the V.P. of the pure solvent.
Reason :
​Some of the solute ​molecules will occupy some surface area of the solution.​Tendency of the solvent particles to go into the vapour phase slightly decreases.Hence vapour pressure of the pure solvent is more than that of the solution.