4.16 Vapor Pressure

Chemical Concept Demonstrated: Vapor presssure

Demonstration:

1. Three universal gas law apparati (see demonstration 4.15) are needed: one using a 2-L flask, one using a 1-L flask, and one using a 500-mL flask.
2. 5-10 mL of acetone is injected into the 1-L flask. Determine the pressure.
3. The same operation is repeated in the 500 mL and 2-L flasks.
4. Determine the vapor pressure of the acetone as the temperature increases.

Observations:

    The pressure of the gas inside the three different flasks is the same, given enough time.  As the temperature increases, the vapor pressure of the acetone increases.

Explanations:

    A liquid in a closed container comes to equilibrium (with respect to vapor pressure) when the rate of condensation is equal to the rate of evaporation.  Because the rates themselves are independent of surface area, the total vapor pressure in the container is independent of surface area.

    Vapor molecules are molecules that have overcome various intermolecular forces to leave the liquid phase.  When heat is applied to the liquid, more molecules gain the energy required to become vapor.  This increase in the rate of evaporation shifts the equilibrium of the vapor pressure, causing more molecules to participate in evaporation and condensation.  As a result, the vapor pressure increases.