Question

Two beakers of capacity $500mL$ were taken. One of these beakers, labelled as “a”, was filled with $400mL$ water whereas the beaker labelled “b” was filled with $400mLof2M$ solution of $NaCl$. At the same temperature both the beakers were placed in closed containers of same material and same capacity as shown in the given figure.


At a given temperature, which of the following statement is correct about the vapour pressure of pure water and that of $NaCl$ solution.

• A. vapour pressure is equal in both the containers.
  
• B. vapour pressure in container (a) is more than that in container (b).
  
• C. vapour pressure in container (a) is less than that in container (b).
  
• D. vapour pressure in container (b) is twice the vapour pressure in container (a).
  

Answer 1

The correct option is B vapour pressure in container (a) is more than that in container (b).


Effect of solute on the vapour pressure of the solvent :

When a non-volatile solute is added to a solvent, the vapour pressure of the solvent decreases because as we add a non-volatile solute, the tendency of the solvent molecules to evaporate decreases.

As a result, fewer molecules change from liquid to gas phase and thus, the vapour pressure decreases. Therefore, we can say that the vapour pressure of a pure solvent is greater than the vapour pressure of a solution containing a non-volatile solute.

Comparing the given solutions

According to the question given,
Beaker “a” contains only water while beaker “b” contains $NaCl$ (a non-volatile solute) dissolved in water. Thus, due to addition of non-volatile solute $NaCl$, vapour pressure in container (a) is more than that in container (b).

Hence, the correct answer is option (A).