Intermolecular forces determine vapor pressure and boiling point of the mixture Z

Steps:

• X and Y have identical boiling points, so they have comparable intermolecular forces and vapor pressures.
• Mixing forms Z, where average intermolecular forces exceed the average of those in X and Y.
• Stronger forces in Z reduce molecular escape rate, lowering vapor pressure at a given temperature.
• Lower vapor pressure of Z implies higher boiling point than X or Y at the same pressure.

Why B is correct:

• Stronger intermolecular forces decrease vapor pressure, as fewer molecules overcome attractions to enter the vapor phase (Clausius-Clapeyron relation).

Why the others are wrong:

• A: Not enough information; stronger forces in Z suggest possible exothermic mixing but do not confirm enthalpy change without interaction details.
• C: Stronger forces increase boiling point, opposite of the deduction.

Final answer: B