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An aqueous solution is 1 molal in \( K I \) Which change will cause the vapour pressure of the solution to increase?

1. Addition of \( N a C l \)
2. Addition of \( N a_{2} S O_{4} \)
3. Addition of 1 molal \( K I \)
4. Addition of water

Correct answer: Addition of water

Solution

For a nonvolatile solute like KI, adding more solute lowers vapour pressure by reducing the mole fraction of water. Adding water dilutes the solution, increases the mole fraction of water, and therefore increases vapour pressure.

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