Question

At 35ºC, the vapor pressure of pure water is 42.2 torr. Calculate the vapor pressure of water at this temperature in a solution that has a concentration of 6.5 M of NaCl. The density of the solution is 1.09 g/mL

Answer 1

Basis: 1 Liter of solution

1L= 1000ml

Since molarity of NaCl is 6.5, i.e 6.5 moles of NaCl are there in 1 Liter of solution

Moles of NaCl= 6.5 , moles= mass/molar mass

mass of NaCl= 6.5* molar mass= 6.5*58.5= 380.25 gm

mass of NaCl solution = Volume of solution* density of the solution =1000ml*1.09gm/ml= 1090gm

mass of Water in the solution = total mass of solution-mass of NaCl= 1090-380.25 =709.75 gm

Moles of water= mass/molar mass= 709.75/18 gmoles = 39.43 gmoles

total moles of solution = moles of water+ moles of NaCl=6.5+39.43= 45.93 moles

mole fraction of water= moles of water/total moles= 39.43/45.93= 0.858

Vapor pressure of water in the solution = pure component vapor pressure of water* mole fraction of water in the solution==42.2*0.858 Torr=36.40 Torr