Science, Tech, Math › Science How to Use Raoult's Law to Calculate Vapor Pressure Change Share Flipboard Email Print Robert Nickelsberg/Contributor/Getty Images Science Chemistry Basics Chemical Laws 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 Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated March 01, 2019 This example problem demonstrates how to use Raoult's 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 °C.The vapor pressure of pure H2O at 39.8 °C is 54.74 torrThe density of H2O at 39.8 °C is 0.992 g/mL. Solution Raoult's Law can be used to express the vapor pressure relationships of solutions containing both volatile and nonvolatile solvents. Raoult's Law is expressed byPsolution = ΧsolventP0solvent wherePsolution is the vapor pressure of the solutionΧsolvent 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 = 36+8+48 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 molΧsolution = nwater/(nwater + nglycerin)Χsolution = 18.63/(18.63 + 1.78)Χsolution = 18.63/20.36Χsolution = 0.91 Find the Vapor Pressure of the Solution Psolution = ΧsolventP0solventPsolution = 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.