Lowering of Vapor Pressure — Overview & Examples

We're exploring colligative properties. They're physical properties that change when a solute dissolves in a solvent. But, they're extensive properties. So, they only depend on the amount of solute.

We've already explored boiling point elevation. We observed that solute addition lowers the vapor pressure. Boiling happens when the vapor pressure equals the atmospheric pressure. So, to reach the new boiling temperature, we have to add more energy. Typically, we increase the temperature via heat transfer.

Why Does Vapor Pressure Lower?

To understand vapor pressure lowering, consider surface tension. The molecules on a liquid's surface experience adhesion and cohesion. Van Der Waal's interactions cause adhesion. For the surface molecules to vaporize, they must overcome Van Der Waal's interactions. When we add a solute, we increase the electrostatic interactions. So, it requires more energy for a molecule to enter the gas phase.

Let's consider water, the universal solvent. In pure water, hydrogen bonding causes adhesion. What happens when we add an ionic salt, like sodium chloride ($NaCl$)? The ionic bond dissociates into ions. Some of the ions will be on the solution's surface. So, there are fewer water molecules on the surface that can vaporize. Also, some of the water molecules now experience ion-dipole interactions. They are stronger than hydrogen bonds. So, it's more difficult for the water to vaporize. We've lowered the vapor pressure.

We can also consider thermodynamics. We've increased the number of electrostatic interactions. So, we've caused the energy to be more dispersed. Energy dispersion is entropy! So, we created a positive entropy change. Now, our system is more stable.

Raoult's Law

To find the vapor pressure change, we use Raoult's law. The formula is $P_{A}=X_{A}P^{o}_{A}$.

• $P_{A}$ is the new vapor pressure. We call it partial pressure.

• $X_{A}$ is the mole fraction of the solvent solution.

• Remember, its formula is $X_{A}=\frac{mols_{solvent}}{mols_{total}}$. The mole fraction is moles divided by moles. So, it's unitless.

• $P^{o}_{A}$ is the vapor pressure of the pure solvent.

To find the change in the vapor pressure, we use the formula $\Delta P=P^{o}_{A}-P_{A}$.

(Video) Raoult's Law - How To Calculate The Vapor Pressure of a Solution

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor illustrates Raoult's law. He works through three example problems. Each problem is more difficult than the last. But, they could easily appear on an exam. If you need practice with Raoult's law this is a great video to watch!