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Unformatted text preview: Vapor Pressure, Raoult’s Law, and Henry’s Law Ch 302, Fall 2009 B. A. Rowland September 2, 2009 Outline • We have now described solutions, classified them on qualitative (electrical conductivity) and quantitative (concentration) bases. • Now, we turn to some of the other properties of solutions we can quantify: – A review of vapor pressure – Vapor pressures of solutions (Raoult’s Law) – Prediction of boiling temperatures with vapor pressure data. – Vapor pressures of gaseous solutions (Henry’s Law) • You should review logarithms and exponentials at this point if you are not familiar with these math topics! What is Vapor Pressure? • Vapor pressure is defined as the pressure of a vapor over a liquid at equilibrium. • Equilibrium is defined as the point at which the rate of vaporization (from the liquid) is equal to the rate of condensation (from the vapor). • The figure to the right demonstrates this concept. – I nitially, the rate of vaporization is greater than the rate of condensation. This leads to the buildup of ethanol molecules in the vapor phase. – After some time has passed, there is enough ethanol in the vapor phase such that the rate of condensation will be the same as the rate of vaporization. At this time, the equilibrium has been Vapor Pressure for Pure Substances • We are concerned with the vapor pressure of liquids and solutions. Most solids have negligible vapor pressures (exceptions are iodine and dry ice, which sublime). • The figure to the right shows the temperature dependence of the vapor pressure of various substances. • Note that the vapor pressure of water is equal to 1 atm (or 760 torr—this is defined as normal atmospheric pressure) at T = 100 o C—the familiar boiling point!...
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 Fall '07
 Madrid
 Vapor, pr essur, Equat ion

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