Raoult’s law explains the vapor pressure of a solvent in an ideal solution using the mole fraction of the solvent. The law assumes an ideal solution and affects the melting and boiling points. The closer a solution is to being ideal, the more accurate the law.
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 mole fraction of the solvent and multiplying it by the vapor pressure of the solvent at a specific temperature when it is in 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 moles of the solvent plus the moles of the solute. A solute is what is 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 higher the energy, so the more particles evaporate. Only the solvent molecules escape the solution because the solute molecules do not have the same tendency to evaporate.
For example, in a salt water solution, the salt is the solute and the water is the solvent. Although salt dissolves in water, it doesn’t turn into a gas while in the water. Only water evaporates.
An equilibrium is established in a closed system. Although the particles still escape the liquid, they have nowhere to go, so they 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 contains only the solvent molecules. In a solution, however, the surface contains molecules of the solvent and the solute. This means fewer particles will escape and the vapor pressure will be lower for a solution than for pure solvent. Raoult’s law explains this change in the escaping particles. Using the mole fraction, it is theoretically possible to determine how many of the particles on the surface of a solution will escape, thus determining the vapor pressure of a solution.
The change in vapor pressure also affects the melting and boiling points. In solutions, the melting point is usually lower and the boiling point higher than in the pure form of the solvent.
Raoult’s law assumes that the solution to be tested is an ideal solution. Since ideal solutions are only theoretical, Raoult’s law is used as the limit 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, while concentrated solutions do not behave exactly as the law suggests.
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