Question

Addition of which of the following will not cause any lowering in vapor pressure of 1 M KI solution?

• A. 1.0 M NaCl solution
• B. 1.0 M KI solution
• C. Both (1) & (2)
• D. Addition of water

Answer 1

Answer: (D)

Addition of water

Answer 2

The vapor pressure lowering is a colligative property, which depends on the number of solute particles in the solution. According to Raoult's Law, the vapor pressure of a solution ($P_s$) is given by $P_s = X_{solvent} \cdot P^0_{solvent}$, where $X_{solvent}$ is the mole fraction of the solvent and $P^0_{solvent}$ is the vapor pressure of the pure solvent. The lowering of vapor pressure ($\Delta P$) is proportional to the mole fraction of the solute ($X_{solute}$).

The initial solution is 1 M KI. Since KI is a strong electrolyte, it dissociates completely into K$^+$ and I$^-$ ions:

KI(aq) → K$^+$ (aq) + I$^-$ (aq)

So, 1 mole of KI produces 2 moles of ions. Therefore, a 1 M KI solution has an effective concentration of 2 M solute particles.

Let's analyze each option:

1. 1.0 M NaCl solution: NaCl is also a strong electrolyte and dissociates completely into Na$^+$ and Cl$^-$ ions:

   NaCl(aq) → Na$^+$ (aq) + Cl$^-$ (aq)

   So, a 1.0 M NaCl solution also has an effective concentration of 2 M solute particles. When a 1.0 M NaCl solution is added to a 1.0 M KI solution, the total volume increases, but the overall effective concentration of solute particles in the mixture remains approximately 2 M (assuming additive volumes and similar densities for dilute solutions). Since the effective concentration of solute particles does not significantly change, the mole fraction of the solvent remains relatively constant. Therefore, the vapor pressure of the solution will likely not change significantly from its initial value.

2. 1.0 M KI solution: Adding a 1.0 M KI solution to an existing 1.0 M KI solution means adding more of the same solution. The concentration of the solute (KI) remains 1 M, and thus the effective concentration of solute particles remains 2 M. Therefore, the vapor pressure of the solution will not change from its initial value.

3. Both (1) & (2): Since both options (1) and (2) individually result in no significant change in the vapor pressure, this option would also imply no lowering of vapor pressure.

4. Addition of water: Adding water to a 1.0 M KI solution will dilute the solution. This means the concentration of KI will decrease, and consequently, the effective concentration of solute particles will decrease. A decrease in solute concentration leads to an increase in the mole fraction of the solvent. According to Raoult's Law, an increase in the mole fraction of the solvent will cause the vapor pressure of the solution to increase. An increase in vapor pressure definitely means it will not cause any lowering.

Conclusion:

All three scenarios (addition of 1.0 M NaCl solution, addition of 1.0 M KI solution, and addition of water) can be interpreted as not causing a lowering of the vapor pressure. However, the addition of water results in an increase in vapor pressure, which is the most distinct effect.