Final answer:
At equilibrium in a closed flask with a volatile sample, the rate of condensation equals the rate of evaporation, which is a dynamic equilibrium state with constant vapor pressure and continuous exchange of molecules between the phases.
Step-by-step explanation:
When a volatile sample in a closed flask is at equilibrium, it is the rate of condensation that equals the rate of evaporation. This scenario describes a dynamic equilibrium, where the processes of vaporization and condensation continue at equal rates, maintaining a constant vapor pressure of the system. It is important to understand that at equilibrium, the actual amounts (moles) of the liquid and gaseous molecules do not necessarily have to be equal; rather, it is the rates at which the liquid vaporizes and the formed vapor condenses that are equal. Similarly, Avogadro's Law pertains to the relationship between the volume and quantity (moles) of a gas, stating that equal volumes of any gas at the same temperature and pressure contain equal numbers of particles, but it does not directly address the state of equilibrium in a closed system.