Question

A liquid is placed in an evacuated, sealed flask. When will this closed system reach dynamic equilibrium?

When all of the liquid has evaporated to form a vapor
When liquid and vapor are present but evaporation stops
When the rate of evaporation equals the rate of condensation
When the volume of liquid equals the volume of vapor

Answer 1

If we ignore temperature equilibrium (since no data given), then dynamic equilibrium when the rate of evaporation equals the rate of condensation over a finite period of time and under constant temperature.

This is because at *dynamic* equilibrium, evaporation and condensation are both taking place continuously. If the rates are equal, then the mass of vapour and mass of liquid phases do not change, hence equilibrium. Dynamic because changes are still taking place.

Answer 2

When the rate of evaporation equals the rate of condensation

Step-by-step explanation:

In a dynamic equilibrium the rate of the forward reaction is equal to the rate of the backward reaction. In evaporation liquid is converted to gas phase and in condensation the gas or vapor is converted to the liquid phase. So dynamic equilibrium is reached when the rate of evaporation equals the rate of condensation.