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Chemistry 105 B
Practice Exam 2
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last Name
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Last 4 Digits of USC ID:____ ____ ____ ____
Dr. Jessica Parr
Lab TA’s Name: _________________________________
Question
1
2
3
4
5
6
7
8
9
10
Points
10
10
8
10
10
14
10
10
8
10
Total
100
Score
Grader
Please Sign Below:
I certify that I have observed all the rules of Academic Integrity while taking this examination.
Signature: _______________________________________________________________
Instructions:
1. You must show work to receive credit.
2. If necessary, please continue your solutions on the back of the preceding page (facing you).
3. YOU MUST use black or blue ink. (No pencil, no whiteout, no erasable ink.)
4. There are 10 problems on 10 pages. Please count them before you begin. A periodic table and
some useful equations can be found on the last page.
5. Good luck!! =)
1
1. (10 pt) Determine whether the following statements are true or false. Correct the false
statements.
a. When covalent oxides are dissolved in water basic solutions result.
True or
False
b. pH + pOH = 14 at all temperatures.
True or
False
c. KaKb = Kw at all temperatures.
True or
False
d. When hydroxide ions are added to a solution buffered with ammonia molecules and
ammonium ions, the ammonia concentration will increase and the ammonium
concentration will decrease.
True or
False
e. The solubility of AgCl will be different in pure water than in a solution with a pH of
4.5.
True or
False
2. (10 pt) Will solutions of the following salts be acidic, basic or neutral? If necessary refer to
the table of Ka and Kb values on the last page.
NaOBr
acidic
basic
neutral
NH4NO3
acidic
basic
neutral
CaF2
acidic
basic
neutral
KI
acidic
basic
neutral
(CH3NH3)2HPO4
acidic
basic
neutral
2
3. (8 pt) List the following 0.2 M solutions in order of increasing pH: NaNO3, KCH3COO,
NH4I, Na3PO4. Refer to the table of Ka and Kb values on the last page if necessary.
<
<
<
4. (10 pt) A mixture of water vapor, ammonia, oxygen and nitrogen gases is initially at
equilibrium:
4 NH3 (g) + 3 O2 (g) ↔ 6 H2O (g) + 2 N2 (g)
ΔH = -1267 kJ
In what direction will the reaction shift (left, right or no change) to reestablish equilibrium, when
the following changes are imposed.
a. oxygen gas is removed
___________________
b. the volume of the container is decreased
___________________
c. the reaction mixture is cooled
___________________
d. ammonia gas is added
___________________
e. Ne gas is added
___________________
3
5. (10 pt) For the following Lewis acid-base pairs, identify the acid and the base and use and
arrow to show how electrons are being donated.
(a)
H
O
]
H
O -
] +
Al
O
H
Al(OH)4-
O
H
(b)
]
H
O ]-
+
O=C=O
HCO3-
4
6. (14 pt) A buffer is composed of 0.45 M formic acid (HCO2H) and 0.55 M potassium formate.
The Ka for formic acid is 1.8 x 10-4.
a. What is the pH of this solution?
b. What is the new pH of the solution if 3.5 g of NaOH is added to 450.0 mL of this buffered
solution? (Assume no change in volume.)
c. Will the pH of the buffered solution change is HCN (Ka = 6.2 x 10-10) is added to the buffered
solution?
5
7. (10 pt) What pH must be maintained by a buffer solution so that no more than 0.010% of the
Mg2+ present in 0.360 M MgCl2 (aq) remains in solution following precipitation of Mg(OH)2 (s)?
Ksp for Mg(OH)2 = 8.9 x 10-12.
6
8. (10 pt) Solid RbF is slowly added to a 1.5 L solution that is 0.010 M CaCl2 and 0.010 M
MgCl2. The Ksp for CaF2 is 4.0 x 10-11 and the Ksp for MgF2 is 6.4 x 10-9.
a. What is the first precipitate to form?
b. What mass of RbF is necessary to begin precipitation of the first precipitate?
c. To successfully separate the metal ions, at least 99% of one ion must be precipitated before the
other metal begins to precipitate. Can Mg2+ and Ca2+ ions in this solution be separated? (Show
work to support your answer).
7
9. (8 pt) Use the appropriate Ksp and Kf values to find the equilibrium constant for the following
reaction:
FeS (s) + 6 CN- (aq) ↔ Fe(CN)64- (aq) + S2- (aq)
Ksp (FeS) = 3.72 x 10-19
Kf [Fe(CN)2] = 1.0 x 1021; Kf [Fe(CN)42-] = 1.0 x 1029; Kf [Fe(CN)64-] = 1.5 x 1035
8
10. (10 pt) Solutions of sodium thiosulfate are used to dissolve unexposed AgBr (Ksp = 5.0 x 1013
) in the developing process for black and white film. What mass of AgBr can dissolve in 1.00
L of 0.500 M Na2S2O3? Ag+ reacts with S2O32- to form a complex ion:
Ag+ (aq) + S2O32- (aq) ↔ Ag(S2O3)22- (aq)
K = 2.9 x 1013
9
I
VIII
1
2
II
III
IV
V
VI
VII
3
4
5
6
7
8
9
10
Li
Be
B
C
N
O
F
Ne
6.941
11
9.012
12
10.81
13
12.01
14
14.01
15
16.0
16
19.00
17
20.18
18
Na
Mg
Al
Si
P
S
Cl
Ar
22.99
24.31
26.98
28.09
30.97
32.07
35.45
39.95
H
1.008
He
4.003
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
39.10
37
40.08
38
44.969
39
47.88
40
50.94
41
51.996
42
54.9380
43
55.847
44
58.9332
45
58.69
46
63.546
47
65.377
48
69.72
49
72.59
50
74.9216
51
78.96
52
79.90
53
83.80
54
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
85.47
55
87.62
56
88.91
57
91.22
72
92.91
73
95.94
74
(99)
75
101.1
76
102.9
77
106.4
78
107.9
79
112.4
80
114.8
81
118.7
82
121.8
83
127.6
84
126.9
85
131.3
86
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
132.9
87
137.3
88
138.9
89
178.5
104
180.9
105
183.85
106
186.2
107
190.2
108
192.2
109
195.09
197.0
200.6
204.4
207.2
209.0
(209)
(210)
(222)
Fr
Ra
Ac
Rf
Db
Sg
Bh
Hs
Mt
(223)
226.0
227.0
(261)
(262)
(263)
(262)
(265)
(268)
Lanthanides
Actinides
x=
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
140.1
90
140.9
91
144.2
92
(145)
93
150.4
94
151.96
95
157.3
96
158.9
97
162
98
164.9
99
167.3
100
168.9
101
173.0
102
175
103
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
232.0
231.0
238.0
237.0
(244)
(243)
(247)
(247)
(251)
(252)
(257)
(258)
(259)
(26)
[ ]
⎛ A ⎞
⎟⎟
pH = pK a + log⎜⎜
[
HA
]
⎠
⎝
−
− b ± b − 4ac
2a
2
Acid
Ka
Base
Kb
HF
6.6 x 10-4
C6H5NH2
7.4 x 10-10
H2C2O4
5.4 x 10-2
NH2OH
9.1 x 10-9
HC2O4-
5.3 x 10-5
(C2H5)3N
5.2 x 10-4
HOBr
2.5 x 10-9
NH3
1.8 x 10-5
H3PO4
7.1 x 10-3
C17H19O3N
7.4 x 10-7
H2PO4-
6.3 x 10-8
C9H7N
6.3 x 10-10
HPO42-
4.2 x 10-13
C5H5N
1.5 x 10-9
HCH3COO
1.8 x 10-5
CH3NH2
4.2 x 10-4
HCN
6.2 x 10-10
NH2NH2
8.5 x 10-7
H2O2
2.2 x 10-12
C2H5NH2
4.3 X 10-4
HOCl
2.9 x 10-8
(CH3)3N
6.3 x 10-5
10
11