Monday, November 25, 2019
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 à °C.The vapor pressure of pure H 2O at 52.0 à °C is 102.1 torrThe density of H2O at 52.0 à °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 à §solventP0solvent wherePsolution is the vapor pressure of the solutionà §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) ââ â 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 molà §solution nwater/(nwater nsolute)à §solution 43.87/(43.87 1.18)à §solution 43.87/45.08à §solution 0.97 ï » ¿Step 2 Find the vapor pressure of the solutionPsolution à §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.
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