Raoults Law Definition in Chemistry

Raoults law is a chemical law that relates the the vapor pressure of a  solution is dependent on the mole fraction of a solute added to solution.Raoults Law is expressed by the formula:Psolution ÃŽ §solventP0solventwherePsolution is the vapor pressure of the solutionÃŽ §solvent is mole fraction of the solventP0solvent is the vapor pressure of the pure solventIf more than one solute is added to the solution, each individual solvents component is added to the total pressure. Raoults law is akin to the ideal gas law, except for solution. The ideal gas law assumes ideal behavior in which the intermolecular forces between dissimilar molecules equals forces between similar molecules. Raoults law assumes the physical properties of the components of a chemical solution are identical. Deviations From Raoult's Law If there are adhesive or cohesive forces between two liquids, there will be deviations from Raoults law. There is negative deviation when the vapor pressure is lower than expected from the law. This occurs when forces between particles are stronger than those between particles in pure liquids. This behavior is observed in a mixture of chloroform and acetone. Here, hydrogen bonds cause the deviation. Another example of negative deviation is in a solution of hydrochloric acid and water. Positive deviation occurs when the cohesion between similar molecules exceeds adhesion between unlike molecules. The result is higher than expected vapor pressure. Both components of the mixture escape solution more readily than if the components were pure. This behavior is observed in mixtures of benzene and methanol and mixtures of chloroform and ethanol. Sources Raoult, F. M. (1886). Loi gà ©nà ©rale des tensions de vapeur des dissolvants (General law of vapor pressures of solvents), Comptes rendus, 104 : 1430-1433.Rock, Peter A. (1969). Chemical Thermodynamics. MacMillan. p.261 ISBN 1891389327.