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Module 4: Liquid Characteristics - Vapor Pressure
Vaporization
Vapor Pressure
Dew Point
Practice Problems
Objective
- Calculate the concentration of a gas in equilibrium with the liquid phase using the vapor pressure.
Liquids can vaporize and evaporate. A liquid at any temperature will
have some molecules with more kinetic energy than other molecules. The
molecules with higher kinetic energy will be able to escape the intermolecular
attractive forces in the liquid and enter the gas phase (see Figure
1 
A, Liquid Vaporization).
Therefore, in the case of liquids some vaporization occurs at all temperatures as long as there is contact between the liquid phase and the gas phase. Vaporization increases as the liquid temperature increases due to the increased number of molecules with the necessary kinetic energy level to escape (or be vaporized).
As long as the liquid is exposed to a gas, vaporization will continue until equilibrium is established or until the liquid has vaporized entirely. In the case of puddles of water on the pavement after a rainstorm, equilibrium is usually not established since there is a readily available supply of fresh air blowing across the puddles to remove the vaporized molecules. Accordingly, the liquid in the puddles is entirely vaporized. However, if the humidity in the air is high, the puddle may establish an equilibrium condition with the air and therefore, there will be no net reduction in the mass of the puddle after equilibrium is reached.
If the liquid is in a closed container, the amount of gas surrounding the liquid is limited. Some of the molecules that vaporize into the gas phase will reenter the liquid phase due simply to the random movement of the molecules. In a short time, the escape and reentry of the molecules will reach an equilibrium level. At equilibrium, the number of molecules vaporizing into the gas phase is balanced by an equal number of molecules reentering the liquid phase (see Figure 1 B, Equilibrium Vapor Pressure).
Vapor pressure is the concentration of the material in the gas stream at this equilibrium condition. In accordance with Dalton's law of partial pressures, the vapor pressure expresses the concentration of the vaporized species in the gas stream. For more information about partial pressure and different ways to express gas concentrations, see Module 2, lesson on Concentrations.
Vapor pressure is useful in the air pollution control field for a variety of reasons.
- Since vapor pressure is proportional to the component in the gas
phase, (i) it can be used to determine the gas concentration, and (ii)
it provides a convenient form to display the concentration of a vaporized
species in a gas stream.
- It can be used to determine the dew
point of a gas.
- It can be used to determine the driving force for gas absorption
by mass transfer. (A lesson on absorption and mass transfer is presented
later in this Module.)
- It indicates the rate of evaporation of a compound.
Example Problem 1.
Calculate the Concentration from the Vapor Pressure
What is the concentration (in ppm) of isopropyl alcohol (IPA) vapor that is in equilibrium with isopropyl alcohol liquid? Use a vapor pressure of 10 mm Hg for IPA and a total pressure of 760 mm Hg.
Solution:
- Calculate the concentration of IPA in the vapor phase expressed as
a mole fraction.
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- Calculate the concentration of IPA in the vapor phase expressed as
ppm.
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For more information about expressing gas concentrations in different formats including parts per million, see the lesson on Concentrations in Module 2.
The dew point of a compound is important in air pollution control work. The dew point is the temperature at which a vapor begins to condense at a constant pressure. Tables are available for determining the dew point given the vapor pressure.
A vapor can be converted to a liquid and removed from the gas stream by lowering the temperature below the dew point. Typical applications include removal of water vapor in evaporative coolers and removal of condensable organic compounds in condensation or refrigeration systems.
The temperature of the gas stream can be maintained above the dew point to prevent condensation. Condensation of acid gases or water vapor may damage or impair equipment such as ducts or fabric filters.
Practice Problems
Vapor Pressure
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.