Raoult vapor pressure depression

Content: colligative properties, concentration, data analysis, mathematical derivation, solutions

Level: advanced

Reference: François-Marie Raoult, "General Law of the Vapor Pressure of Solvents", Comptes Rendus, 104, 1430-3 (1887)

Notes: François-Marie Raoult (1830-1901) made extensive measurements of phenomena we now call colligative properties, particularly the depression of freezing points and vapor pressures of solutions. (Freezing point depression is the subject of another set of Classic Calculations exercises.) The relationship we call Raoult's law says that the vapor pressure of a solvent in a solution is equal to its mole fraction times its vapor pressure as a pure liquid.

psolution = xApsolvent .
This relationship is a reasonable approximation for mixtures of similar liquids or for solvents in which fairly small quantities of non-volatile solvents are dissolved, and it can be derived from a model in which solvent-solute interactions are the same as solvent-solvent interactions. Raoult's investigations of vapor pressure included studies of vapor pressure depression, a colligative property with a proportionality constant he treated empirically.

Note: The expression to be derived in exercise 3,

Δp = xBp ,
implies Raoult's law rather directly. Simply subtract this expression from the vapor pressure of the pure solvent to obtain Raoult's law.

Pedagogical note: In teaching Raoult's law, it is important to discuss its limitations. In particular, solvent-solute interactions can make the "law" a poor approximation if the solution is not dilute unless the solvent and solute are chemically similar molecules. For some strong opinions on this subject, see Stephen J. Hawkes, "Raoult's law is a deception", J. Chem. Educ. 72, 204ff (1995) and (somewhat moderated) Stephen J. Hawkes, "Strategic consequences from errors in Raoult's law paper", J. Chem. Educ. 73, 41ff (1996).

Solutions: To download solutions, go to:

Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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