Version 041 – Exam 2 – Fakhreddine – (52395)
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001
10.0 points
Suppose the reaction
H
2
(g) + I
2
(g)
⇀
↽
2 HI(g)
has an equilibrium constant
K
c
= 49 and the
initial concentrations of H
2
, I
2
and HI are 0.50
M, 0.50 M and 0.00 M, respectively. What is
the correct value for the final concentration of
HI(g)?
1.
0.599 M
2.
0.778 M
correct
3.
0.389 M
4.
0.250 M
5.
0.438 M
6.
0.219 M
Explanation:
K
c
= 49
[H
2
]
ini
= 0
.
50 M
[I
2
]
ini
= 0
.
50 M
[HI]
ini
= 0
.
0 M
H
2
(g)
+
I
2
(g)
⇀
↽
2 HI (g)
ini, M
0.5
0.5
0
Δ, M
−
x
−
x
2
x
eq, M
0
.
5
−
x
0
.
5
−
x
2
x
K
c
=
[HI]
2
[H
2
] [I
2
]
= 49
(2
x
)
2
(0
.
50
−
x
)
2
= 49
2
x
0
.
50
−
x
= 7
2
x
= 3
.
5
−
7
x
9
x
= 3
.
5
x
= 0
.
389
[HI] = 2
x
M = 0
.
778 M
002
10.0 points
K
= 8, for the gas phase reaction
N
2
O
4
⇀
↽
2 NO
2
.
If 2 moles of N
2
O
4
are put into a 1 liter
vessel, the number of moles of NO
2
at equilib
rium (
n
x
) can be calculated from the equation
1.
8 =
n
2
x
2
−
n
x
2
correct
2.
8 =
n
x
2
−
n
x
2
3.
8 =
n
2
x
2
−
n
x
4.
8 =
n
2
x
2
5.
8 =
2
n
2
x
2
−
n
x
Explanation:
Since the problem asks you to put the equa
tion in terms of
n
x
, which we are told is equal
to [NO
2
], we need a slight modification to our
normal way of working this problem. Working
this problem the normal way tells us that final
[NO
2
] is equal to 2
n
x
. Continuing with this
method will not give us the correct answer.
Normal way:
N
2
O
4
→
2 NO
2
(g)
Initial
2
0
Change
−
n
x
+2
n
x
Equilibrium 2
−
n
x
2
n
x
In the modified way, all
n
x
coefficients are
divided in half to give the appropriate [NO
2
]
=
n
x
equality. This is ok to do because we’ve
kept the correct ratio in our change step;
i.e.
,
[NO
2
] must be twice as big as [N
2
O
4
].
Modified way:
N
2
O
4
→
2 NO
2
(g)
Initial
2
0
Change
−
0
.
5
n
x
+
n
x
Equilibrium 2
−
0
.
5
n
x
n
x
Continuing with the modified way . . .
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Version 041 – Exam 2 – Fakhreddine – (52395)
2
K
= 8 =
[NO
2
]
2
[N
2
O
2
]
=
n
2
x
2
−
0
.
5
n
x
=
n
2
x
2
−
n
x
2
003
5.0 points
In a solution labeled “0.10 M HNO
3
,” which
of the following is CORRECT?
1.
[HNO
3
] = 0.050 M, [H
3
O
+
] = 0.050 M,
[NO
−
3
] = 0.050 M
2.
[H
3
O
+
] = 0.10 M, [OH
−
] = 1
.
0
×
10
−
7
M
3.
[HNO
3
] = 0.10 M
4.
[H
3
O
+
] = 0.09 M, [NO
−
3
] = 0.010 M
5.
[H
3
O
+
] = 0.10 M, [NO
−
3
] = 0.10 M
cor
rect
Explanation:
004
5.0 points
Which of the following is the
best
chemical
definition of pure (or neutral) water at any
temperature?
1.
water that has a pH equal to ZERO
2.
water where the concentration of H
+
is
equal to the concentration of OH
−
correct
3.
water that has a pH exactly equal to 7.00
4.
water that has been freshly pumped from
a well
5.
water that has a pH equal to
K
w
6.
water that has a pOH slightly above 7.00
Explanation:
You were probably able to narrow this one
down to either “water where the concentra
tion ...” or “water that has a pH exactly equal
to 7.00.”
The second of these is incorrect because pH
varies with temperature since
K
w
varies, but
the concentrations of H and OH will still be
equal to each other.
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 Spring '07
 Holcombe
 Chemistry, Equilibrium, pH, Reaction, Kc

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