Raoults Law Example Problem - Vapor Pressure Change

Raoult's Law Example Problem - Vapor Pressure Change This example problem demonstrates how to use Raoults Law to calculate the change in vapor pressure by adding a strong electrolyte to a solvent. Raoults Law relates the vapor pressure of a solution on the mole fraction of the solute added to a chemical solution. Vapor Pressure Problem What is the change in vapor pressure when 52.9 g of CuCl2 is added to 800 mL of H2O at 52.0 Â °C.The vapor pressure of pure H 2O at 52.0 Â °C is 102.1 torrThe density of H2O at 52.0 Â °C is 0.987 g/mL. Solution Using Raoult's Law 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 Step 1 Determine the mole fraction of solutionCuCl2 is a strong electrolyte. It will completely dissociate into ions in water by the reaction:CuCl2(s) → Cu2(aq) 2 Cl-This means we will have 3 moles of solute added for every mole of CuCl2 added.From the periodic table:Cu 63.55 g/molCl 35.45 g/molmolar weight of CuCl2 63.55 2(35.45) g/molmolar weight of CuCl2 63.55 70.9 g/molmolar weight of CuCl2 134.45 g/molmoles of CuCl2 52.9 g x 1 mol/134.45 gmoles of CuCl2 0.39 molTotal moles of solute 3 x (0.39 mol)Total moles of solute 1.18 molmolar weightwater 2(1)16 g/molmolar weightwater 18 g/moldensitywater masswater/volumewatermasswater densitywater x volumewatermasswater 0.987 g/mL x 800 mLmasswater 789.6 gmoleswater 789.6 g x 1 mol/18 gmoleswater 43.87 molÃŽ §solution nwater/(nwater nsolute)ÃŽ §solution 43.87/(43.87 1.18)ÃŽ §solution 43.87/45.08ÃŽ §solution 0.97 ï » ¿Step 2 Find the vapor pressure of the solutionPsolution ÃŽ §solventP0solventPsolution 0.97 x 102.1 torrPsolution 99.0 torr ï » ¿Step 3 Find the change in vapor pressureChange in pressure is Pfinal - POChange 99.0 torr - 102.1 torrchange -3.1 torr Answer The vapor pressure of the water is reduced by 3.1 torr with the addition of the CuCl2.