Which of the following statements correctly describes the effect of adding a nonvolatile solute to a solvent according to Raoult's Law?

• A. The vapor pressure of the solution is lower than that of the pure solvent.
• B. The vapor pressure of the solution depends only on the temperature and not on the presence of solute.
• C. The vapor pressure of the solution is higher than that of the pure solvent.
• D. The vapor pressure of the solution remains unchanged compared to the pure solvent.

Answer: (A)

Verified step by step guidance

1. Recall Raoult's Law, which states that the vapor pressure of a solvent in a solution, \(P_{solvent}\), is proportional to the mole fraction of the solvent, \(X_{solvent}\), times the vapor pressure of the pure solvent, \(P^0_{solvent}\). Mathematically, this is expressed as: \[P_{solvent} = X_{solvent} \times P^0_{solvent}\]
2. Understand that when a nonvolatile solute is added to a solvent, the mole fraction of the solvent, \(X_{solvent}\), decreases because the total number of moles in the solution increases while the moles of solvent remain the same.
3. Since \(X_{solvent}\) is less than 1 in the solution (compared to pure solvent where \(X_{solvent} = 1\)), the vapor pressure of the solvent in the solution, \(P_{solvent}\), will be lower than the vapor pressure of the pure solvent, \(P^0_{solvent}\).
4. Recognize that the vapor pressure depends on both temperature and the composition of the solution. Adding a nonvolatile solute changes the composition, thus affecting the vapor pressure.
5. Conclude that the correct description according to Raoult's Law is: 'The vapor pressure of the solution is lower than that of the pure solvent.'