
Concept explainers
Calculate the vapor pressure of water over each of the following solutions of glycerol, C3H8O3, at 28°C (vapor pressure of pure water=28.35 mm Hg). Glycerol can be assumed to be nonvolatile.
(a) percent glycerol by mass=30.6%
(b) 2.74 m
(c) mole fraction of glycerol=0.188

(a)
Interpretation:
To calculate vapor pressure of water if mass percent of glycerol is 30.6 %.
Concept introduction:
Formula to calculate moles of a component is −
Moles=MassMolar mass
Mole fraction of a component in a solution can be found out by dividing moles of the component by summation of mole of all components in the solution.
Mole fraction (X)=Moles of componentSummation of moles of all components
Raoult’s law states that vapor pressure of solvent in a mixture is equal to the product of mole fraction of solvent and vapor pressure of pure solvent.
Psolvent=Xsolvent×P0solvent
Where, Psolvent is vapor pressure of solvent in a mixture, Xsolvent is mole fraction of solvent, and P0solvent is vapor pressure of pure solvent.
Answer to Problem 34QAP
Vapor pressure of water if mass percent of glycerol is 30.6% is 26.08 mm Hg.
Explanation of Solution
Given:
Vapor pressure of pure water = 28.55 mm Hg at 28 °C.
Given mass percent of glycerol is 30.6%. It means in 100 g solution mass of glycerol is 30.6 g
Mass of solvent(water)=100 g−30.6 g=69.4 g
So, mass of water is 69.4 g.
Moles of each component of the solution are to be calculated as follows:
Mass of glycerol =30.6 g
Molar mass of glycerol =92.09 g mol−1
Moles of glycerol=Mass of glycerolMolar mass of glycerol=30.6 g92.09 g mol−1=0.33 mol
So, moles of glycerol is 0.33 mol.
Mass of water =69.4 g
Molar mass of water =18.02 g mol−1
Moles of water=Mass of waterMolar mass of water=69.4 g18.02 g mol−1=3.85 mol
So, moles of water is 3.85 mol.
Calculation of mole fraction of water
Mole fraction of water(Xwater)=Moles of waterMoles of water+Moles of glycerol=3.85 mol3.85 mol+0.33 mol=3.85 mol4.18 mol=0.92
So, mole fraction of water is 0.92.
Raoult’s law states that vapor pressure of solvent in a mixture is equal to the product of mole fraction of solvent and vapor pressure of pure solvent.
Psolvent=Xsolvent×P0solvent
Where, Psolvent is vapor pressure of solvent in a mixture, Xsolvent is mole fraction of solvent and P0solvent is vapor pressure of pure solvent.
Mole fraction of water Xwater=0.92
Vapor pressure of pure solvent =28.35 mm Hg
Vapor pressure of water can be calculated as follows:
Pwater=Xwater×P0water=0.92×28.35 mm Hg=26.08 mm Hg
Hence, vapor pressure of water is 26.08 mm Hg.

(b)
Interpretation:
To calculate vapour pressure of water if molality of the solution is 2.74 m.
Concept introduction:
Molality is one way to define the concentration of solution. It is the ratio of moles of solute to mass of solvent in kilogram.
Formula of molality is-
Molality=Moles of soluteMass of solvent(kg)
Raoult’s law states that vapor pressure of solvent in a mixture is equal to product of mole fraction of solvent and vapor pressure of pure solvent.
Psolvent=Xsolvent×P0solvent
Where, Psolvent is vapor pressure of solvent in a mixture, Xsolvent is mole fraction of solvent and P0solvent is vapor pressure of pure solvent
Answer to Problem 34QAP
If molality of the solution is 2.74 m, vapor pressure of water will be 26.93 mm Hg.
Explanation of Solution
Given:
Vapor pressure of pure water = 28.55 mm Hg at 28 °C.
Given the molality is 2.74 m. So mass of solvent is 1000 g(1 kg)
Moles of solute (glycerol) can be calculated as follows:
Molality=Moles of soluteMass of solvent(kg)2.74 m=Moles of solute1 kgMoles of solute=2.74 mol kg−1 kg=2.74 mol
So, moles of solute (glycerol) is 2.74 mol
Moles of solvent can be calculated as follows:
Molar mass of water is =18.02 g mol−1
Mass of solvent (water) =1000 g
Moles of solvent(water)=1000 g18.02 g mol−1=55.5 mol
So, moles of solvent is 55.5 mol.
Mole fraction of solvent can be calculated as follows:
Mole fraction of solvent(Xwater)=Moles of solvent (water)Moles of solvent (water)+Moles of solute (glycerol)=55.5 mol55.5 mol+2.74 mol=0.95
So, mole fraction of solvent (water) is 0.95.
Raoult’s law states that vapor pressure of a solvent in a mixture is equal to the product of mole fraction of solvent and vapor pressure of pure solvent.
Psolvent=Xsolvent×P0solvent
Where, Psolvent is vapor pressure of solvent in a mixture, Xsolvent is mole fraction of solvent and P0solvent is vapor pressure of pure solvent.
Mole fraction of water Xwater=0.95
Vapor pressure of pure solvent =28.35 mm Hg
Vapor pressure of water can be calculated as follows:
Pwater=Xwater×P0water=0.95×28.35 mm Hg=26.93 mm Hg
Hence, vapor pressure of water is 26.93 mm Hg.

(c)
Interpretation:
To calculate vapor pressure of water if mole fraction of glycerol is 0.188.
Concept introduction:
For a solution containing solute (glycerol) and solvent (water), summation of mole fraction is equal to one.
Xethylene glycol+Xwater=1
Raoult’s law states that vapor pressure of solvent in a mixture is equal to the product of mole fraction of solvent and vapor pressure of pure solvent.
Psolvent=Xsolvent×P0solvent
Where, Psolvent is vapor pressure of solvent in a mixture, Xsolvent is mole fraction of solvent and P0solvent is vapor pressure of pure solvent
Answer to Problem 34QAP
Vapor pressure of water if mole fraction of glycerol is 0.188 is 23.02 mm Hg.
Explanation of Solution
Given:
Vapor pressure of pure water = 28.55 mm Hg at 28 °C.
Given mole fraction of glycerol (X glycerol) is 0.188 and vapor pressure of pure water is 28.35 mm Hg
Addition of mole fraction of all components of a solution is equal to one.
Xethylene glycol+Xwater=1Xwater=1−Xethylene glycol=1−0.188=0.812
So, mole fraction of water is 0.812.
Raoult’s law states that vapor pressure of solvent in a mixture is equal to the product of mole fraction of solvent and vapor pressure of pure solvent.
Psolvent=Xsolvent×P0solvent
Where, Psolvent is vapor pressure of solvent in a mixture, Xsolvent is mole fraction of solvent and P0solvent is vapor pressure of pure solvent.
Mole fraction of water Xwater=0.812
Vapor pressure of pure solvent =28.35 mm Hg
Vapor pressure of water can be calculated as follows:
Pwater=Xwater×P0water=0.812×28.35 mm Hg=23.02 mm Hg
Hence, vapor pressure of water is 23.02 mm Hg.
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