A system of greater disorder of molecules is more probable. The disorder of molecules is reflected by the entropy of the system. A liquid vapourizes to form a more disordered gas. When a solute is present, there is additional contribution to the entropy of the liquid due to increased randomness. As the entropy of solution is higher than that of pure liquid, there is weaker tendency to form the gas. Thus, a solute (non-volatile) lowers the vapour pressure of a liquid, and hence a higher boiling point of the solution.
Similarly, the greater randomness of the solution opposes the tendercy to freeze. In consequence, a lower temperature must be reached for achieving the equilibrium between the solid (frozen solvent) and the solution. The elevation in boiling point `(DeltaT_(b))` and depression in freezing point `(DeltaT_(f))` of a solution are the colligative properties which depend only on the concentration of particles of the solute and not their identity. For dilute solutions, `(DeltaT_(b))` and `(DeltaT_(f))` are proportional to the molarity of the solute in the solution.
To aqueous solution of `Nal`, increasing amounts of solid `Hgl_(2)` is added. The vapour pressure of the solution

Options

A. decreases to a constant value
B. increases to a constant value
C. increases and then decreases
D. remains constant as `Hgl_(2)` is sparingly soluble in water

Answer

D

Explanation

`2NaI_(2) + HgI_(2) rarr Na_(2)Hgl_(4)`
As `HgI_(2)` is added, the number of molecules will decrease due to the formation of `Na_(2)Hgl_(4)`. Therefore, pressure will increase. As more `HgI_(2)` is added, pressure will remain constant (and`Nal` is consumed) after that as `HgI_(2)` is sparingly soluble.