Vapor pressure is best defined as?

Vapor pressure is best understood as the force exerted by a vapor that is in equilibrium with its corresponding liquid at a given temperature. This concept highlights the tendency of a liquid's molecules to escape into the vapor phase; as more molecules vaporize, they create pressure in the space above the liquid.

At equilibrium, the rate of evaporation and the rate of condensation are equal, leading to a stable vapor pressure specific to that liquid and its current temperature. Hence, option C accurately captures this definition, emphasizing the relationship between vapor and liquid phases.

The other options do not correctly describe vapor pressure: the rate of evaporation refers to how quickly a liquid turns into vapor, mass per unit volume describes density, and the temperature at which vapor condenses pertains to the condensation point rather than the pressure exerted by the vapor. These distinctions clarify why the correct understanding of vapor pressure aligns with the concept of force exerted by the vapor.