
Concept explainers
(a)
Interpretation:
The mole fraction of heptane and hexane in liquid phase has to be given.
Concept Introduction:
Mole fraction: Mole fraction of a substance in a solution is the number of moles of that substance divided by the total number of moles of all substances present. The formula is,
(a)

Answer to Problem 12.85QE
The mole fraction of hexane is
The mole fraction of heptane is
Explanation of Solution
Given,
Weight of hexane =
Weight of heptane =
Vapor pressure of hexane =
Vapor pressure of heptane =
The moles of heptane and hexane are calculated from their molar masses respectively.
Moles of hexane=
Moles of heptane=
The mole fraction of hexane in the liquid phase is calculated as,
Mole fraction of hexane=
Mole fraction of hexane=
Mole fraction of hexane=
The mole fraction of heptane is calculated from the mole fraction of hexane.
Mole fraction of heptane=
Mole fraction of heptane=
The mole fraction of hexane is
The mole fraction of heptane is
(b)
Interpretation:
The vapor pressures of hexane and heptane above the solution have to be given.
Concept Introduction:
The equilibrium between a liquid and its vapor produces a characteristic vapor pressure for each substance that depends on the temperature. The lowering of the vapor pressure is caused by a lesser ability of the solvent to evaporate, so equilibrium is reached with a smaller concentration of the solvent in the gas phase. The vapor pressure of a solution is expressed using Raoult’s law:
The vapor pressure of the solvent
(b)

Answer to Problem 12.85QE
The vapor pressure of hexane is
The vapor pressure of heptane is
Explanation of Solution
Given,
Weight of hexane =
Weight of heptane =
Vapor pressure of hexane =
Vapor pressure of heptane =
The moles of hexane are calculated from its molar mass.
Moles of hexane=
The mole fraction of hexane in the liquid phase is calculated as,
Mole fraction of hexane=
Mole fraction of hexane=
Mole fraction of hexane=
The vapor pressure of hexane is given as,
Vapor pressure of hexane=
Vapor pressure of hexane=
Vapor pressure of hexane=
The moles of heptane are calculated from its molar mass.
Moles of heptane=
The mole fraction of heptane is calculated from the mole fraction of hexane.
Mole fraction of heptane=
Mole fraction of heptane=
The vapor pressure of heptane is given as,
Vapor pressure of heptane=
Vapor pressure of heptane=
Vapor pressure of heptane=
The vapor pressure of hexane is
The vapor pressure of heptane is
(c)
Interpretation:
The mole fraction of heptane and hexane in vapor phase has to be given.
Concept Introduction:
Refer to part (a) and (b).
(c)

Answer to Problem 12.85QE
The mole fraction of hexane in vapor pressure is
The mole fraction of heptane in vapor pressure is
Explanation of Solution
The vapor pressure of hexane
The vapor pressure of heptane is
The total vapor pressure is
The mole fraction of hexane and hexane are calculated as,
Mole fraction=
Mole fraction of hexane=
Mole fraction of heptane=
The mole fraction of hexane in vapor pressure is
The mole fraction of heptane in vapor pressure is
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Chapter 12 Solutions
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