Version 125 – Exam 3 – Mccord – (90540)
1
This
printout
should
have
25
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
McCord
CHs302
001
10.0 points
M
2+
is added to a solution of 0
.
0063 mol
/
L
X

and
0
.
0016
mol
/
L
Z
3

.
MX
2
has
K
sp
= 5
.
1
×
10

12
, while M
3
Z
2
has
K
sp
=
6
.
5
×
10

29
.
What percentage of the Z

3
is
still in solution (not precipitated) when the
MX
2
just starts to precipitate?
1. 3.68
2. 3.16
3. 3.88
4. 5.03
5. 3.01
6. 6.38
7. 6.22
8. 5.84
9. 8.36
10. 10.9
Correct answer: 10
.
9%.
Explanation:
K
sp MX
2
= 5
.
1
×
10

12
[X

] = 0
.
0063 mol
/
L
K
sp M
3
Z
2
= 6
.
5
×
10

29
[Z

3
] = 0
.
0016 mol
/
L
The two relevant equilibria here are
MX
2
(s)
⇀
↽
M
2+
+ 2X

K
sp
= [M
2+
] [X

]
2
M
3
Z
2
(s)
⇀
↽
3M
+2
+ 2Z
3

K
sp
= [M
2+
]
3
[Z
3

]
2
MX
2
would precipitate when
K
sp
= [M
2+
] [X

]
2
5
.
1
×
10

12
= [M
2+
] (0
.
0063 mol
/
L)
2
[M
2+
] = 1
.
28496
×
10

7
mol
/
L
This corresponds to a [Z
3

] of
K
sp
= [M
2+
]
3
[Z
3

]
2
6
.
5
×
10

29
= (1
.
28496
×
10

7
mol
/
L)
3
[Z
3

]
2
[Z
3

] = 0
.
000175034 mol
/
L
The percentage of [Z
3

] still in solution is
0
.
000175034 mol
/
L
0
.
0016 mol
/
L
(100 %) = 10
.
9%
002
10.0 points
When
ammonium
chloride
is
added
to
NH
3
(aq),
1.
the equilibrium concentration of NH
3
(aq)
decreases.
2.
the pH of the solution does not change.
3.
the
K
b
increases.
4.
the pH of the solution increases.
5.
the pH of the solution decreases.
correct
Explanation:
003
10.0 points
Consider the fractional composition dia
gram for phosphoric acid where p
K
a1
= 2.12,
p
K
a2
= 7.21, and p
K
a3
= 12.68.
0.2
0.4
0.6
0.8
1.0
2
4
6
8
10
12
14
pH
Fractional Composition
What is the structure of the dominant
species at pH 3?
1.
PO
3

4
2.
HPO
2

4
3.
H
3
PO
4
4.
H
2
PO

4
correct
Explanation:
To the left of 2
.
12, the red graph represent
ing H
3
PO
4
is dominant.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Version 125 – Exam 3 – Mccord – (90540)
2
Between 2
.
12 and 7
.
21, the blue graph rep
resenting H
2
PO

4
is dominant.
Between 7
.
21 and 12
.
68, the orange graph
representing HPO
2

4
is dominant.
To the right of 12
.
68, the green graph rep
resenting PO
3

4
is dominant.
004
10.0 points
What is the molar solubility of CuBr in 0.5 M
NaBr? The
K
sp
is 4
.
2
×
10

8
.
1.
4
.
20
×
10

8
2.
8
.
40
×
10

8
correct
3.
4
.
20
×
10

7
4.
3
.
48
×
10

3
5.
2
.
05
×
10

4
Explanation:
005
10.0 points
You have available the following reagents as
0.10 M aqueous solutions: NaOH, HCl, HCN
(p
K
a
= 9.31), aniline (p
K
b
= 9.13), HNO
2
(p
K
a
= 3.25), and CH
3
NH
2
(p
K
b
= 3.34).
Which two reagents would you use to make
the best buffer with a pH of 10.70?
1.
HCl and CH
3
NH
2
correct
2.
NaOH and HNO
2
3.
NaOH and HCN
4.
HCl and NaOH
5.
HCl and aniline
Explanation:
The pKa for CH
3
NH
+
3
is 10.66 which is the
closest pKa to the desired buffer pH of 10.70.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Summer '10
 McCord
 pH, Solubility

Click to edit the document details