Explanation χ a 1 862069 0 137931 the gas phase mole

At a given temperature, pure A has a vapor
pressure of 140 torr and pure B has a vapor
pressure of 60 torr. At this temperature, we
make a solution in which the mole fractions
of A and B are equal. We then allow this
solution to reach equilibrium with its vapor.
Which of the following statements about the
composition of the VAPOR is TRUE?
4. The mole fraction of A in the vapor is 70
torr.
5. This question cannot be answered with-
out knowing the molecular weights of A and
B.
6. Equal numbers of moles of A and B are
present in the vapor.
7. All of the other answers are true.
8. The mole fraction of A in the vapor is less
than 0.5.
9. All of the other answers are false.
Explanation:
For CH
2
Cl
2
,
P
0
= 133 torr MW = 85 g / mol
For CH
2
Br
2
,
P
0
= 11 torr MW = 174 g / mol
This is a combination of Raoult’s Law and
Dalton’s Law of Partial Pressures. The an-
swer does not depend on what the masses are,
as long as they are equal. You can choose any
mass you like, but to speed up calculations, it
is convenient to choose the mass the same as
one of the molecular weights given, so that the
number of moles for one of the components is
exactly ONE.
So, for argument’s sake, choose 85 g to be the
mass of each of the components. That way
you have:
(85 g CH
2
Cl
2
)
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1 mol CH
2
Cl
2
85 g CH
2
Cl
2
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