How to Use Raoults Law to Calculate Vapor Pressure Change

How to Use Raoult's Law to Calculate Vapor Pressure Change This example problem demonstrates how to use Raoults Law to calculate the change in vapor pressure by adding a nonvolatile liquid to a solvent. Problem What is the change in vapor pressure when 164 g of glycerin (C3H8O3) is added to 338 mL of H2O at 39.8 Â °C.The vapor pressure of pure H2O at 39.8 Â °C is 54.74 torrThe density of H2O at 39.8 Â °C is 0.992 g/mL. Solution Raoults Law can be used to express the vapor pressure relationships of solutions containing both volatile and nonvolatile solvents. Raoults Law is expressed byPsolution ÃŽ §solventP0solvent wherePsolution is the vapor pressure of the solutionÃŽ §solvent is mole fraction of the solventP0solvent is the vapor pressure of the pure solvent Determine the Mole Fraction of Solution molar weightglycerin (C3H8O3) 3(12)8(1)3(16) g/molmolar weightglycerin 36848 g/molmolar weightglycerin 92 g/molmolesglycerin 164 g x 1 mol/92 gmolesglycerin 1.78 molmolar weightwater 2(1)16 g/molmolar weightwater 18 g/moldensitywater masswater/volumewatermasswater densitywater x volumewatermasswater 0.992 g/mL x 338 mLmasswater 335.296 gmoleswater 335.296 g x 1 mol/18 gmoleswater 18.63 molÃŽ §solution nwater/(nwater nglycerin)ÃŽ §solution 18.63/(18.63 1.78)ÃŽ §solution 18.63/20.36ÃŽ §solution 0.91 Find the Vapor Pressure of the Solution Psolution ÃŽ §solventP0solventPsolution 0.91 x 54.74 torrPsolution 49.8 torr Find the Change in Vapor Pressure Change in pressure is Pfinal - POChange 49.8 torr - 54.74 torrchange -4.94 torr Answer The vapor pressure of the water is reduced by 4.94 torr with the addition of the glycerin.