Question

To aqueous solution of $NaI$, increasing amounts of solid $Hg{I}_2$ is added. The vapor pressure of the solution-

• A. Decreases to a constant value.
• B. Increases to a constant value.
• C. Increases first and then decreases.
• D. Remains constant because $Hg{I}_2$ is sparingly soluble in water.

Answer 1

The correct option is B Increases to a constant value.

The reaction is $2N{a}^{+}\left(aq\right)+2I^{-}\left(aq\right)+Hg{I}_2\left(s\right)\to 2N{a}^{+}\left(aq\right)+Hg{I}_4^{2-}\left(aq\right)$
As the reaction progresses, the number of moles of particles decreases from $4(2N{a}^{+}+2I^{-})$ to $3(2N{a}^{+}+Hg{I}_4^{2-})$ . This decreases the colligative properties and increases the vapour pressure. The vapour pressure increases to a constant value till NaI is completely consumed.