Version 304 – 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 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.
900 torr
2.
780 torr
3.
120 torr
correct
4.
330 torr
5.
660 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
002
10.0 points
Which gas would you expect to have the
largest value for the van der Waals constant
“a”?
1.
CH
4
2.
Ne
3.
NH
3
correct
4.
He
Explanation:
003
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.
9.25 mol
2.
9.48 mol
3.
18
.
52 mol
correct
4.
7.51 mol
5.
20.5 mol
Explanation:
28 mol

9
.
48 mol = 18
.
52 mol
004
10.0 points
Nitric acid is produced commercially by the
Ostwald process.
In the first step, ammonia
is oxidized to nitric oxide:
4NH
3
(g) + 5O
2
(g)
→
4NO(g) + 6H
2
O(g)
Assume this reaction is carried out in the
apparatus diagramed below.
2.00 L NH
3
0.500 atm
1.00 L O
2
1.50 atm
The stopcock between the two reaction con
tainers is opened, and the reaction proceeds
using proper catalysts. Calculate the partial
pressure of NO after the reaction is complete.
Assume 100% yield for the reaction, assume
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Version 304 – Exam 3 – mccord – (50970)
2
the final container volume is 3.00 L, and as
sume the temperature is constant.
1.
0.250 atm
2.
0.333 atm
correct
3.
0.500 atm
4.
1.50 atm
5.
0.400 atm
Explanation:
First, calculate the partial pressures of NH
3
and O
2
after the stopcock is opened (and
before the reaction). Use Boyle’s Law... new
volume is 3 L so that...
P
NH
3
= (2L/3L)(1/2atm) = 1/3 atm
P
O
2
= (1L/3L)(3/2atm) = 1/2 atm
You must have at least 5/4 more moles of
O
2
than NH
3
for complete reaction.
That
means you need 5/12 atm of O
2
.
You have
more than enough (0.5 atm = 6/12 atm) so
the oxygen is in excess.
The easy part... You make the SAME num
ber of moles of NO as you use NH
3
, so the final
partial pressure of the NO will be the same as
the initial partial pressure of the NH
3
which
is 1/3 atm or 0.333 atm.
005
10.0 points
The density of the vapor of allicin, a compo
nent of garlic, is 1
.
14 g
·
L
−
1
at 125
◦
C and 175
Torr. What is the molar mass of allicin?
1.
162 g
·
mol
−
1
correct
2.
21
.
6 g
·
mol
−
1
3.
50
.
8 g
·
mol
−
1
4.
869 g
·
mol
−
1
5.
273 g
·
mol
−
1
Explanation:
T
= 125
◦
C + 273.15 K = 398
.
15 K
P
= (175 Torr)
1 atm
760 Torr
= 0
.
230263 atm
ρ
= 1
.
14 g
/
L
The ideal gas law is
P V
=
n R T
n
V
=
P
R T
with unit of measure mol/L on each side.
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 Fall '07
 Fakhreddine/Lyon
 Chemistry, NH3, mol

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