Which term describes the pressure of a vapor in equilibrium with its liquid in a closed container?

• A. Pressure
• B. Boiling Point
• C. Vapor Pressure
• D. Molecular Weight

Answer: (C)

Vapor pressure is the pressure exerted by the vapor when it is in dynamic equilibrium with its liquid in a closed container. At that equilibrium, molecules evaporate from the liquid and condense back from the vapor at equal rates, so the pressure in the space above the liquid remains constant. This pressure depends on temperature and the liquid’s properties—the higher the temperature, the greater the vapor pressure. Boiling point is related but different: it’s the temperature at which the vapor pressure equals the surrounding pressure, not the pressure of the vapor itself at equilibrium. Molecular weight isn’t the term used for this phenomenon.