Chem 3411 Solution Set 5 - Mindanao State University - Iligan Institute Of Technology - 2010 Page 2
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14CHEM 3411, Fall 2010
Solution Set 5
Additionally, we know the solution only contains 2-propanol and the unknown with moles fraction of χ
and χ
p
u
respectively, and therefore χ
+ χ
= 1. Combining this with our expression for χ
gives and expression for χ
in
p
u
p
u
terms of measured pressures.
∗
χ
= 1
p
/p
u
p
p
Next, we’ll use the definition of mole fraction to relate χ
to the moles of unknown by
u
n
u
χ
=
u
n
+ n
u
p
where n
and n
are the moles of unknown and 2-propanol respectively.
u
p
This equation can be solved for n
u
χ
n
u
p
n
=
u
1
χ
u
We also know moles is equal to mass over molecule weight.
m
u
n
=
u
M
w,u
m
p
n
=
p
M
w,p
Equating these two expressions for n
, inserting the expression for n
, and rearranging for M
gives
u
p
w,u
1
χ
u
M
= m
w,u
u
m
p
χ
u
M
w,p
Lastly, we can insert our earlier expression for χ
and simplify the resulting equation to
u
(
)
M
1
w,p
M
= m
1
w,u
u
p
p
m
1
p
∗
p
p
60.1 & g/ mol
1
= 8.69 g
1
¨ ¨
250 & g
49.62
kPa
1
¨ ¨
50.00
kPa
= 272.79 g/ mol
Solution
M
= 273 g/ mol
w,u
2
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