Which of the following pairs of substances has the higher vapor pressure at 298 K listed first?

• A. Pure water and aqueous NaCl solution (water listed first)
• B. Pure ethanol and aqueous ethanol solution (ethanol listed first)
• C. Aqueous ethanol solution and pure ethanol (solution listed first)
• D. Aqueous NaCl solution and pure water (solution listed first)

Answer: (B)

Verified step by step guidance

1. Understand that vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. Pure substances generally have higher vapor pressures than solutions containing solutes because solutes lower the vapor pressure (Raoult's Law).
2. Recall Raoult's Law, which states that the vapor pressure of a solvent in a solution, \(P_{solvent}\), is given by \(P_{solvent} = X_{solvent} \times P^\circ_{solvent}\), where \(X_{solvent}\) is the mole fraction of the solvent and \(P^\circ_{solvent}\) is the vapor pressure of the pure solvent.
3. Apply this concept to the pairs: for pure water vs. aqueous NaCl solution, the pure water has a higher vapor pressure because the presence of NaCl lowers the mole fraction of water and thus lowers the vapor pressure.
4. Similarly, for pure ethanol vs. aqueous ethanol solution, the pure ethanol has a higher vapor pressure because the addition of water reduces the mole fraction of ethanol, lowering the vapor pressure of the solution compared to pure ethanol.
5. Conclude that in each pair, the pure substance has the higher vapor pressure at 298 K, so the correct order lists the pure substance first.