Science, Tech, Math Science Raoult's Law Definition in Chemistry Share Flipboard Email Print Raoult's Law can be used to calculate vapor pressure. Steve McAlister, Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry in Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Anne Marie Helmenstine, Ph.D. Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Updated January 09, 2019 Raoult's 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.Raoult's Law is expressed by the formula:Psolution = ΧsolventP0solventwherePsolution is the vapor pressure of the solutionΧsolvent 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 solvent's component is added to the total pressure. Raoult's 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. Raoult's 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 Raoult's 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 générale 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. Continue Reading