Question

Which of the following changes decrease the vapor pressure of water kept in a sealed vessel?

• A. Decreasing the quantity of water
• B. Adding salt to water
• C. Decreasing the volume of the vessel to one-half
• D. Keeping the temperature of water constant

Answer 1

Answer: (B)

When salt (non-volatile solute) is added to water kept in a sealed vessel, boiling point of the solution will be elevated or increased. So, vapour pressure of water (solution) will be decreased. Lowering of vapour pressure ( $\Delta p=p^{\circ }-p)$ is also proportional to mole fraction of the solute $\left({\chi }_B\right)$ present in the solution.
$\Delta p\propto {\chi }_B[p^{\circ }=$ vapour pressure of pure water
$p=$ vapour pressure of solution $]$