Chem 3411 Solution Set 5 - Mindanao State University - Iligan Institute Of Technology - 2010 Page 10

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CHEM 3411, Fall 2010
Solution Set 5
5
Exercise 5.25b pg 202
Given
Benzene and toluene form nearly ideal solutions. Consider an equimolar solution of benzene and toluene. At 20
C
the varpour pressure of pure benzene and pure toluene are 9.9 kPa and 2.9 kPa, respectively. The solution is boiled
by reducing the external pressure below the vapour pressure.
In terms of given variables, this is written:
T = 20
C
p
= 9.9 kPa
b
p
= 2.9 kPa
t
χ
= χ
= 0.5000
(Equimolar solution)
b
t
Find
Calculate . . .
• (a) the pressure when boiling begins
• (b) the composition of each component in the vapour
• (c) the vapour pressure when only a few drops of liquid remain.
Assume that the rate of vaporization is low enough for the temperature to remain constant at 20
C.
Strategy
For part (a) we’ll start with Raoult’s Law (Equation 5.21 pg 164) that relates the partial pressure of the vapour of a
liquid in a solution p
to the partial pressure of the pure liquid p
by the mole fraction of the liquid in solution χ
.
a
a
a
p
= χ
p
a
a
a
The partial pressures of each component are thereby given as
p
= χ
p
b
b
b
= 0.5000 × 9.9 kPa
= 4.950 kPa
p
= χ
p
t
t
t
= 0.5000 × 2.9 kPa
= 1.450 kPa
10

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