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Vapor Pressure: Definition, Equation & Examples

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  • 0:03 What is Vapor Pressure?
  • 1:17 The Clausius-Clapeyron…
  • 2:06 Clausius-Clapeyron…
  • 3:19 Raoult's Law
  • 4:13 Raoult's Law Example
  • 5:27 Lesson Summary
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Lesson Transcript
Instructor: Nichole Miller

Nichole is a research scientist with a PhD in Materials Science & Engineering.

This lesson explains the definition of vapor pressure, describes how vapor pressure changes as a function of temperature according to the Clausius-Clapeyron equation, and shows how to determine the vapor pressure of a single component in a mixture of liquids using Raoult's Law.

What Is Vapor Pressure?

Water can take the form of a liquid, a solid (ice), or a gas (water vapor). Ice melts to form water, and water evaporates to form water vapor. Think about a water bottle like the one shown here:

Water bottle

What's inside? You can see water and air, but what you may not have realized is that there is also water vapor. In a water bottle, a little bit of the water is constantly evaporating and becoming water vapor. At the same time, some of the water vapor is constantly condensing to become a liquid. At equilibrium, the amount of water evaporating is equal to the amount of water vapor condensing, so the amounts of water and water vapor are constant.

The water vapor exerts a pressure on the water bottle in the same way that air pumped into a tire exerts pressure on the tire. The pressure exerted by the water vapor is the vapor pressure. In more general terms, vapor pressure is the pressure exerted by a gas in equilibrium with the same material in liquid or solid form. Just as distance can be measured in a variety of units (miles, feet, kilometers, etc.), we can also measure vapor pressure with many different units (kPa, atm, bar, mm Hg, torr).

The Clausius-Clapeyron Equation

Let's consider what happens if the temperature of the water bottle increases a little. The water molecules will have more energy to evaporate, so there will be a little more water vapor and a little less water in the bottle at equilibrium at this higher temperature. The amount of pressure exerted by the water vapor will also increase, meaning that the vapor pressure will increase. The Clausius-Clapeyron equation describes how temperature affects vapor pressure.

Clausius-Clapeyron Equation

In this equation, P1 and P2 are the vapor pressures of a material at temperatures T1 and T2, respectively. R is the ideal gas constant (8.314 J/(mol·K)) and Hv is the enthalpy of vaporization of the material, a number that you can look up for common materials. As expected, the equation shows that vapor pressure increases as temperature increases.

Clausius-Clapeyron Equation Example

Let's look at an example using the Clausius-Clapeyron equation.

Given that the vapor pressure of water is 1 atm at its boiling point, 100°C (373 K), and that the enthalpy of vaporization of water is 40,700 J/mol, use the Clausius-Clapeyron Equation to determine the vapor pressure of water at 80°C (353 K).

The Clausius-Clapeyron equation can be rewritten in the following more-convenient form:

Clausius-Clapeyron Equation

We know that T1 = 353 K, T2 = 373 K, P2 = 1 atm, and Hv = 40,700 J/mol. Plugging these values into the equation, we get the following formula:

Application of the Clausius-Clapeyron Equation

Therefore, the vapor pressure of water at 80°C is 0.48 atm.

Raoult's Law: Understanding Vapor Pressure in Mixtures

Sometimes we have to consider vapor pressure in a mixture of liquids. In the ideal case, the tendency of a molecule to escape will not change when it is mixed with another liquid and we can use Raoult's Law to determine the vapor pressure of the components.

Raoult

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