Dalton’s and Raoult’s Laws

Dalton’s Law of Partial Pressure states that “the pressure of a mixture of gases is equal to the sum of the pressures of all of the constituent gases alone.”

Mathematically, this can be represented as:

Pressure Total = Pressure¹ + Pressure² … Pressuren

Dalton’s Law explains that the total pressure above a solution, such as a chlorinated solvent in water, is equal to the sum of the partial pressures of the parts. This mixture will boil when the vapor pressure above it is equal to ambient pressure. Thus, the boiling point of the solvent/water mixture is depressed below the boiling point of the solvent. For example, tetrachloroethene (PCE) boils at 121 °C in air and at approximately 81 °C when immersed in water or in contact with soil moisture. Similar reductions in boiling points are observed for all chlorinated solvents in contact with water.

Raoult’s law states that “The vapour pressure of a solution is dependent on the vapor pressure of each component and the mole fraction of the component present in the solution”.

It may also be expressed as follows: At a given temperature, the partial vapor pressure of a component in a mixture is equal to the vapor pressure of the pure component at that temperature multiplied by its mole fraction in the mixture.

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