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Chemistry 2080  Spring 2011
Prelim #2
Tuesday April 12th, 7:30 to 9:00 pm
Name:
___________
KEY_______________________
CU ID:
____________________
Last
First
M.I.
Lab:
_______________________________
Lab TA Name:
__________________
Day
Time
Instructions:
Answer all questions in the spaces provided. You have 90 minutes. You must
show all work for full credit. There are 9 pages in this exam, and there is also a separate
equation sheet.
All temperatures are assumed to be 25°C unless otherwise stated.
If you have to solve a quadratic equation
, please write it out first in the standard form, such
as: ax
2
+ bx + c
=
0, and then you can use a calculator to solve it directly.
The following are not allowed at the exam: mp3 players, iPods and similar devices,
headphones or earbuds, cell phones, iPads, Laptop computers, machines that go “Ping”.
There are 6 questions; the points for each are indicated below.
1. _____________/15
2. _____________/16
3. _____________/22
4. _____________/18
5. _____________/15
6. _____________/14
Total
_____________/100
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1)
(15 points)
(a)
Calculate the pH at 25°C of a 1.28x10
7
M
aqueous solution of the strong base Ca(OH)
2
.
Please give
your answer to two decimal places. (You will need to take into account the autoionization of water).
______________________________
Calcium hydroxide is a strong base, therefore it ionizes essentially completely in aqueous solution:
Ca(OH)
2
(s)
∏
Ca
2+
(aq)
+
2 OH

(aq)
As
the solution is so dilute, we cannot ignore the autoionization of water. We therefore have the following
equations:
Material balance:
[Ca
2+
] = 1.28x10
7
M
. . . . . . . (1)
Electroneutrality:
2[Ca
2+
]
+
[H
3
O
+
]
=
[OH

]
. . . . . . . (2)
And the definition of the ionproduct of water:
K
w
=
[H
3
O
+
][OH

]
=
1.00x10
14
. . . . . . . (3)
Substituting from equations (1) and (3) into (2), we get:
2 (1.28x10
7
)
+
[H
3
O
+
]
=
K
w
/[H
3
O
+
]
i.e.
[H
3
O
+
]
2
+
2.56x10
7
[H
3
O
+
]
–
1.00x10
14
=
0
which is a quadratic equation in [H
3
O
+
], so we can use the familiar equation to solve this
quadratic,
whence:
3
O
+
]
=
3.443x10
8
(taking the positive root (the negative root is meaningless))
and thus
pH = 7.46
(
If the autoionization of water is ignored, the value would be
pH = 7.41)
(b)
What is the pH at 25°C of the solution obtained by mixing 50 mL of 2.0 x10
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 Spring '07
 DAVIS,F
 Chemistry

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