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What is Raoult's Law?

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  • Written By: Angie Bates
  • Edited By: John Allen
  • Last Modified Date: 26 November 2016
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Raoult's law is used in chemistry to explain the behavior of solvents when a non-volatile solute is exposed to temperature changes. This law determines the vapor pressure of a solvent at a given temperature in an ideal solution. The pressure can be found by using the solvent's mole fraction and multiplying it by the vapor pressure of the solvent at a specific temperature when it in is its pure form.

A mole fraction is the number of moles of a solvent divided by the total number of moles in the solution. Since a solution is a combination of a solvent and a solute, the total number of moles is the solvent's moles plus the solute's moles. A solute is what is being dissolved, and a solvent is what the solute is dissolved in.

Vapor pressure results from particles in a liquid escaping from the liquid, or evaporating. Particles with higher energy that are on the surface of the liquid can escape. The higher the temperature, the more energy, so the more particles evaporate. Only the solvent's molecules escape from the solution because the solute's molecules do not have the same tendency to evaporate.

For example, in a solution of salt water, salt is the solute and water is the solvent. Although salt dissolves in the water, it does not change into a gas while in the water. Only the water evaporates.

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In a closed system, an equilibrium is established. Although particles still escape the liquid, they have nowhere to go, so they just bounce off the walls of the system and eventually return to the liquid. The moving particles create pressure, called saturated vapor pressure.

In a pure form, the surface of a liquid solvent only contains the solvent's molecules. In a solution, however, the surface contains molecules of the solvent and the solute. This means that fewer particles will escape, and the vapor pressure will be less for a solution than for the pure solvent. Raoult's law accounts for this change in escaping particles. Using the mole fraction, it is theoretically possible to determine how many of the particles on the surface of a solution will be able to escape, thus determining the vapor pressure of a solution.

The change in vapor pressure also effects melting and boiling points. In solutions, the melting point is generally lower and the boiling point higher than in the solvent's pure form.

Raoult's law assumes the solution being tested is an ideal solution. Since ideal solutions are only theoretical, Raoult's law is used as a limiting law. The closer a solution is to being an ideal solution, the more accurate Raoult's law will be when applied to that solution. Extremely dilute solutions behave almost exactly as Raoult's law states, whereas concentrated solutions will not behave quite as the law suggests.

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