Version 373 – Exam 3 – mccord – (50970)
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McCord
CH301
This exam is only for Dr.
McCord’s CH301
classes.
001
10.0 points
A mixture of oxygen and helium is 92.3% by
mass oxygen.
It is collected at atmospheric
pressure (687 torr).
What is the partial pressure of oxygen in
this mixture?
1.
333 Torr
2.
446 Torr
3.
299 Torr
4.
414 Torr
correct
5.
688 Torr
Explanation:
Assume you have 100 g of this mixture;
calculate the number of moles:
n
O
2
= (92
.
3 g O
2
)
1 mol O
2
31
.
9988 g O
2
= 2
.
91261 mol O
2
.
n
He
= (7
.
7 g He)
1 mol He
4
.
0026 g He
= 1
.
92375 mol He
.
n
tot
=
n
O
2
+
n
He
= 2
.
91261 mol O
2
+ 1
.
92375 mol He
= 4
.
83636 mol gas
Dalton’s Law:
P
O
2
=
P
tot
×
χ
O
2
=
P
tot
×
n
O
2
n
tot
= (687 Torr)
2
.
91261 mol O
2
4
.
83636 mol gas
= 413
.
733 Torr
002
10.0 points
A gas has a volume of 2.00 liters at a tempera
ture of 127
◦
C. What will be the volume of the
gas if the temperature is increased to 327
◦
C?
(Assume the pressure remains constant.)
1.
6.00 liters
2.
3.00 liters
correct
3.
2.00 liters
4.
4.00 liters
Explanation:
T
1
= 127
◦
C + 273 = 400 K
V
1
= 2
.
00 L
T
2
= 327
◦
C + 273 = 600 K
Charles’ Law relates the volume and ab
solute (Kelvin) temperature of a sample of
gas:
V
1
T
1
=
V
2
T
2
V
2
=
V
1
T
2
T
1
=
(2
.
00 L) (600 K)
400 K
= 3
.
00 L
003
10.0 points
A mixture of CO, CO
2
and O
2
is contained
within a 275 mL flask at 0
◦
C. If the total pres
sure is 780 torr, the CO has a partial pressure
of 330 torr and the CO
2
has a partial pressure
of 330 torr, what is the partial pressure of O
2
?
1.
120 torr
correct
2.
780 torr
3.
900 torr
4.
660 torr
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Version 373 – Exam 3 – mccord – (50970)
2
5.
330 torr
Explanation:
P
total
= 780 torr
P
CO
= 330 torr
P
CO
2
= 330 torr
P
total
=
P
CO
+
P
CO
2
+
P
O
2
P
O
2
=
P
total

P
CO

P
CO
2
= 780 torr

330 torr

330 torr
= 120 torr
004
10.0 points
If a real gas has a volume that is larger than
you would predict based on the ideal gas law,
then the intermolecular forces for that gas are
dominated by
1.
attractive forces
2.
repulsive forces
correct
3.
gravitational forces
4.
neither attractive nor repulsive forces
5.
compression forces
Explanation:
If the volume of the real gas is larger than
you would predict based on the ideal gas law,
then the compression factor Z
>
1.
This
means the intermolecular forces are domi
nated by repulsions between the gas particles
(atoms or molecules).
005
10.0 points
The gauge in this problem is an absolute pres
sure gauge so assume the pressure is exactly
as stated in this problem (do not adjust it).
An oxygen tank kept at 20
◦
C contains 28
moles of oxygen and the gauge reads 31 atm.
After two weeks, the gauge reads 10
.
5 atm.
How much oxygen was used during the two
week period?
1.
20.5 mol
2.
7.51 mol
3.
9.48 mol
4.
9.25 mol
5.
18
.
52 mol
correct
Explanation:
28 mol

9
.
48 mol = 18
.
52 mol
006
10.0 points
For an ideal gas, pressure and volume are
directly proportional.
1.
False
correct
2.
True
Explanation:
Boyle’s law,
V
=
k
1
P
, relates the volume
and pressure of a sample of gas at constant
temperature.
Pressure and volume are in
versely proportional.
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 Fall '07
 Fakhreddine/Lyon
 Chemistry, Torr

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