dsj294 – hw06 – Holcombe – (53570)
1
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printout
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have
25
questions.
Multiplechoice questions may continue on
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before answering.
PLEASE NOTE: Due to the tolerances on
numerical problems (your answer must be
within ONE PERCENT of the correct answer
unless otherwise noted), if a problem requires
the quadratic formula to find the solution, DO
NOT make any assumptions (i.e. USE THE
QUADRATIC FORMULA). Also, be sure to
enter excess significant figures to assure that
you are not penalized for rounding.
001
10.0 points
Solution A contains HCl and has an H
+
con
centration of 10
−
3
M. Solution B contains
CH
3
COOH and has an H
+
concentration of
10
−
5
M. Which solution is more acidic and
which solution has a greater pH?
1.
Solution A is more acidic, but Solution B
has a greater pH.
correct
2.
Solution B is more acidic and has a greater
pH.
3.
Solution A is more acidic and has a greater
pH.
4.
Both solutions have identical acidities and
pH.
5.
Solution B is more acidic, but Solution A
has a greater pH.
Explanation:
[H
+
]
A
= 10
−
3
M
[H
+
]
B
= 10
−
5
M
pH =
−
log[H
+
]
The solutions have different concentrations
of H
+
.
Therefore, they will have different
acidities and pH. The more concentrated will
be more acidic and have a lower pH.
002
10.0 points
The buildup of lactic acid in muscles causes
pain during extreme physical exertion.
The
K
a
for lactic acid (C
2
H
5
OCOOH) is 8
.
4
×
10
−
4
. Calculate the pH of a 0
.
06 M solution
of lactic acid.
1.
2.149
2.
3.533
3.
2.174
correct
4.
1.926
5.
2.043
6.
2.297
7.
6.348
8.
11.851
Explanation:
K
a
= 8
.
4
×
10
−
4
[C
2
H
5
OCOOH] = 0
.
06
You MUST use the quadratic formula to
solve this one. If you use the approximation,
your answer will be more than 5% off.
C
2
H
5
OCOOH + H
2
O
⇀
↽
Initial
0.06
Final
0
.
06
−
x
C
2
H
5
OCOO
−
+ H
3
O
+
Initial
−
−
Final
x
x
K
a
=
bracketleftbig
C
2
H
5
OCOO
−
bracketrightbig bracketleftbig
H
3
O
+
bracketrightbig
[C
2
H
5
OCOOH]
=
x
2
0
.
06
−
x
= 8
.
4
×
10
−
4
x
2
+ 8
.
4
×
10
−
4
x
−
5
.
04
×
10
−
5
= 0
To solve the quadratic equation,
x
=
−
b
±
√
b
2
−
4
ac
2
a
Since
b
2
−
4
ac
=
(
−
8
.
4
×
10
−
4
)
2
−
4 (1)
(
−
5
.
04
×
10
−
5
)
= 2
.
02
×
10
−
4
,
then
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dsj294 – hw06 – Holcombe – (53570)
2
x
=
(
−
8
.
4
×
10
−
4
)
±
√
2
.
02
×
10
−
4
2 (1)
= (0
.
00669
,
−
0
.
00753)
,
x
=
bracketleftbig
H
3
O
+
bracketrightbig
= 0
.
00669, so
pH =
−
log
bracketleftbig
H
3
O
+
bracketrightbig
=
−
log(0
.
00669)
= 2
.
174
003
10.0 points
The pH of 0
.
1 M HClO
2
(chlorous acid)
aqueous solution was measured to be 1
.
2.
What is the value of p
K
a
for chlorous acid?
1.
1.20
2.
1.40
3.
3.91
4.
2.57
5.
0.11
6.
0.96
correct
Explanation:
M
= 0
.
1 M
pH = 1
.
2
Analyzing the reaction with molarities,
HClO
2
+ H
2
O
⇀
↽
H
3
O
+
+ ClO
−
2
0
.
1
−
0
0
−
x
−
x
x
0
.
1
−
x
−
x
x
[H
3
O
+
] = [ClO
−
2
] = 10
−
pH
= 10
−
1
.
2
= 0
.
0630957 mol
/
L
The
K
a
is
K
a
=
[H
3
O
+
][ClO
−
2
]
[HClO
2
]
=
(0
.
0630957)
2
0
.
1
−
0
.
0630957
= 0
.
107876
and the p
K
a
is
p
K
a
=
−
log(0
.
107876) = 0
.
967077
.
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 Spring '07
 bronzewook
 pH, Acids, Ka HCl

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