Version 028 – Exam 2 – fakhreddine – (51615)
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001
10.0points
The equilibrium constant for the reaction
2 SO
2
(g) + O
2
(g)
⇀
↽
2 SO
3
(g)
has the value
K
= 30000 at 700 K. Find the
value of
K
for the reaction
SO
3
(g)
⇀
↽
SO
2
(g) +
1
2
O
2
(g)
at the same temperature.
1.
3
.
95
×
10
−
10
2.
6
.
32
×
10
−
6
3.
2
.
64
×
10
−
7
4.
8
.
23
×
10
−
8
5.
5
.
77
×
10
−
3
correct
6.
5
×
10
−
13
Explanation:
For the first equation
2 SO
2
(g) + O
2
(g)
⇀
↽
2 SO
3
(g)
K
1
=
P
2
SO
3
P
2
SO
2
·
P
O
2
= 30000
.
For the second reaction
SO
3
(g)
⇀
↽
SO
2
(g) +
1
2
O
2
K
=
P
SO
2
·
P
1
/
2
O
2
P
SO
3
=
1
√
K
1
=
1
√
30000
= 0
.
0057735
002
10.0points
For gases initially contained in a 1.0 L vessel
and the reaction
2 NO
2
(g)
⇀
↽
N
2
O
4
(g)
Δ
H
◦
=

37
.
7 kJ/mol,
which of the following would NOT cause the
equilibrium to shift to the left and convert
some N
2
O
4
to NO
2
?
I. addition of 1 mol He(g) to the vessel;
II. isothermally decreasing the volume of
the container to 0.425 L;
III. addition of 1 mol of N
2
O
4
(g);
IV. increasing the temperature;
1.
IV only
2.
II only
3.
I and II
correct
4.
III only
5.
II and IV
6.
I and III
7.
I only
Explanation:
003
10.0points
Assume that five weak acids, identified only
by numbers (1, 2, 3, 4 and 5), have the follow
ing ionization constants.
Ionization
Acid
Constant
K
a
value
1
1
.
0
×
10
−
3
2
3
.
0
×
10
−
5
3
2
.
6
×
10
−
7
4
4
.
0
×
10
−
9
5
7
.
3
×
10
−
11
A 0.10 M solution of which acid would have
the highest pH?
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Version 028 – Exam 2 – fakhreddine – (51615)
2
1.
1
2.
4
3.
2
4.
5
correct
5.
3
Explanation:
K
a
=
[H
+
] [A
−
]
[HA]
HA
⇀
↽
H
+
+ A
−
A high pH means that the [H
+
] concentra
tion is low.
(Remember that values greater
than 7 are basic!)
The larger values of
K
a
means that there is more [H
+
] so you would
expect these solutions to be more acidic;
i.e.
,
have smaller pH’s.
The smaller
K
a
values
mean less [H
+
] in the solution, so higher pH’s.
The acid with the smallest
K
a
(#5) will have
the highest pH and lowest [H
+
] concentration.
004
10.0points
Consider the following reaction:
PCl
5
(g)
→
PCl
3
(g) + Cl
2
(g)
.
At equilibrium, [PCl
5
] = 2.00 M and [PCl
3
]
= [Cl
2
] = 1.00 M. If suddenly 1.00 M PCl
3
(g)
and Cl
2
(g) are added, calculate the new equi
librium concentration of PCl
5
(g).
1.
1.86 M
2.
2.17 M
3.
2.25 M
4.
2.08 M
5.
2.81 M
correct
6.
1.53 M
Explanation:
K
=
(1)(1)
2
= 0
.
5
after the addition of 1 M for PCl
3
and Cl
2
the
reaction must shift to the left to reestablish
equilibrium.
K
=
(2

x
)
2
(2 +
x
)
= 0
.
5
which converts to 0 =
x
2

4
.
5
x
+ 3
The quadratic formula will yield two an
swers for
x
.
x
= 0
.
8139 or 3
.
686
The 3.686 answer is not physically possible
and therefore
x
= 0
.
8139.
[PCl
5
] = 2 +
x
= 2
.
81 M
005
10.0points
A diprotic acid H
2
A has values of
K
a1
=
1
.
0
×
10
−
6
and
K
a2
= 1
.
0
×
10
−
10
.
What is
the [A
2
−
] in a 0.10 M solution of H
2
A?
1.
1
.
0
×
10
−
10
M
correct
2.
3
.
2
×
10
−
4
M
3.
3
.
2
×
10
−
6
M
4.
0.20 M
5.
0.10 M
Explanation:
006
10.0points
The pH of 0
.
015 M HNO
2
(nitrous acid) aque
ous solution was measured to be 2
.
63. What
is the value of p
K
a
of nitrous acid?
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 Spring '11
 FATIMA
 pH, NH3, Kc, 0.005 m, Pini

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