Question

Which of the following changes will cause an increase in vapor pressure of 1 molal aqueous KI solution at the same temperature?

• A. Addition of 0.1 molal solution of NaCl
• B. Addition of 0.5 molal solution of Na\(_2\)SO\(_4\)
• C. Addition of water
• D. Addition of 1 molal KI solution

Hint:

To increase the vapor pressure of a solution, add more solvent (e.g., water), not solute.

Answer 1

The Correct Option is C

Step 1: Understanding Vapor Pressure.
Vapor pressure of a solution is determined by the number of solvent molecules available at the surface of the liquid. The addition of non-volatile solute lowers the vapor pressure by forming a solution that reduces the number of solvent molecules at the surface (Raoult’s Law). However, adding water, which is a solvent, increases the total amount of solvent molecules, thereby increasing the vapor pressure.

Step 2: Analyzing the options.
(A) Addition of 0.1 molal solution of NaCl: Incorrect. Adding NaCl increases the ionic content in the solution, which will lower the vapor pressure due to increased solute-solvent interactions.
(B) Addition of 0.5 molal solution of Na\(_2\)SO\(_4\): Incorrect. This also adds more solute, decreasing the vapor pressure.
(C) Addition of water: Correct. Adding more solvent (water) increases the number of solvent molecules, which increases the vapor pressure.
(D) Addition of 1 molal KI solution: Incorrect. This adds more solute, which would decrease the vapor pressure.

Step 3: Conclusion.
The correct answer is (C) Addition of water, as it increases the number of solvent molecules, thus increasing the vapor pressure.