Chemistry 105 B
Exam 2
PLEASE PRINT YOUR NAME IN BLOCK LETTERS
First Letter of
last Name
Name: __________________________________
7/21/08
Last 4 Digits of USC ID:____ ____ ____ ____
Dr. Jessica Parr
Lab TA’s Name: _________________________________
Question
1
2
3
4
5
6
7
8
Points
20
8
18
10
16
8
10
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 9 problems on 9 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. (20 pt) Answer the following multiple choice questions.
i. At 0 oC Kw is 1.2 x 10-15. The pH of pure water at 0 oC is:
a. 7.00
b. 6.88
c. 7.56
d. 7.46
ii. Which of the following is not true for a solution at 25 oC that has a hydroxide concentration of
2.5 x 10-6 M?
a. Kw = 1 x 10-14
b. The solution is acidic
b. The solution is basic
c. The [H+] is 4 x 10-9 M
e. The Kw is independent of what the solution contains.
iii. Which of the following species, when dissolved in H2O, will not produce a basic solution?
a. SO2
b. NH3
c. BaO
d. Ba(OH)2
e. none of these
iv. If the Ka for HCN is 6.2 x 10-10, what is Kb for CN-?
a. 6.2 x 10-24
b. 6.2 x 104
c. 1.6 x 10-5
d. 1.6 x 1023
v. Which of the following aqueous solutions will have the highest pH? For NH3, Kb = 1.8 x 10-5;
for C2H3O2-, Kb = 5.6 x 10-10.
a. 2.0 M NaOH
b. 2.0 M NH3
d. 2.0 M HCl
e. all the same
c. 2.0 M HC2H3O2
vi. Calculate the pH of a 0.10 M solution of Ca(OH)2.
a. 13.3
b. 13.0 M
c. 0.2
d. 0.1
2
vii. For the stepwise dissociation of aqueous H3PO4, which of the following is not a conjugate
acid-base pair?
a. HPO42- and PO43-
b. H3PO4 and H2PO4-
b. H2PO4- and HPO42-
d. H2PO4- and PO43-
e. H3O+ and H2O
viii. The equilibrium constant for the reaction: A- (aq) + H+ (aq) ↔ HA (aq) is defined as:
a. Ka
b. Kb
c. 1/Ka
d. Kw/Kb
e. KwKa
ix. The pH of a 1.0 M aqueous solution of NaCl is:
a. equal to 7.00
b. greater than 7.00
c. less than 7.00
d. not enough information provided
x. The pH of a 1.0 M aqueous solution of NH4Br is:
a. equal to 7.00
b. greater than 7.00
c. less than 7.00
d. not enough information provided
2. (8 pt) The equilibrium constants (Ka) for HCN, HF and HOBr in H2O at 25 oC are 6.2 x 10-10,
7.2 x 10-4, and 2.5 x 10-9, respectively. Give the relative order of base strengths from least to
greatest.
<
<
<
3
3. (18 pt) Answer the following multiple choice questions.
i. Calculate the [H+] concentration in a solution that is 0.10 M in NaF and 0.20 M in HF.
(Ka = 7.2 x 10-4)
a. 0.20 M
b. 7.0 x 10-4 M
c. 1.4 x 10-3 M
d. 3.5 x 10-4 M
ii. Which of the following has the greatest buffering capacity?
a. 5.0 M H2CO3 and 5.0 M NaHCO3
b. 0.5 M H2CO3 and 0.5 M NaHCO3
c. 3.0 M H2CO3 and 3.0 M NaHCO3
d. 0.3 M H2CO3 and 0.3 M NaHCO3
e. all of these will have the same buffering capacity
iii. You have two buffered solutions. Buffered solution 1 consists of 5.0 M HOAc and 5.0 M
NaOAc; buffered solution 2 is made of 0.050 M HOAc and 0.050 M NaOAc. How do the pHs
of the buffered solutions compare?
a. The pH of buffered solution 1 is equal to that of buffered solution 2.
b. The pH of buffered solution 1 is greater than that of buffered solution 2.
c. The pH of buffered solution 2 is greater than that of buffered solution 1.
d. Cannot be determined without the Ka values.
iv. What combination of substances will give a buffered solution that has a pH of 5.05? (Assume
each pair of substances is dissolved in 5.0 L of water.)(Kb for NH3 = 1.8 x 10-5; Kb for C5H5N =
1.7 x 10-9)
a. 1.0 mole NH3 and 1.5 mole NH4Cl
b. 1.0 mole C5H5N and 1.5 mole C5H5NHCl
c. 1.5 mole NH3 and 1.0 mole NH4Cl
d. 1.5 mole C5H5N and 1.0 mole C5H5NHCl
4
v. How many of the following will raise the pH of a weak acid HA in aqueous solution?
I. Addition of water
II. Addition of NaA (s)
IV. Addition of HNO3
V. Addition of KOH
a. 1
b. 2
c. 3
III. Addition of NaCl
d. 4
e. 5
vi. Which of the following will not produce a buffered solution?
a. 100 mL of 0.1 M Na2CO3 and 50 mL of 0.1 M HCl
b. 100 mL of 0.1 M NaHCO3 and 25 mL of 0.2 M HCl
c. 100 mL of 0.1 M Na2CO3 and 75 mL of 0.2 M HCl
d. 50 mL of 0.2 M Na2CO3 and 5 mL of 1.0 M HCl
e. 100 mL of 0.1 M Na2CO3 and 50 mL of 0.1 M NaOH
vii. For ammonia, Kb is 1.8 x 10-5. To make a buffered solution of pH 10.0, the ratio of NH4Cl to
NH3 must be:
a. 1.8 : 1
b. 1: 1.18
c. 0.18 : 1
d. 1: 0.18
e. none of these
viii. For a solution equimolar in HCN and NaCN, which statement is false?
a. This is an example of the common ion effect.
b. The [H+] is larger than it would be if only the HCN was in solution.
c. The [H+] is equal to the Ka.
d. Addition of more NaCN will shift the acid dissociation equilibrium of HCN to the left.
e. Addition of NaOH will increase [CN-] and decrease [HCN].
5
ix. A weak acid, HF, is in solution with dissolved sodium fluoride, NaF. If HCl is added, which
ion will react with the extra hydrogen ions from the HCl to keep the pH from changing?
a. OH-
b. Na+
c. F-
d. Na-
e. none of these
4. (10 pt) Which of the following mixtures would result in a buffered solution when 1.0 L of
each of the two solutions are mixed? (Circle yes or no)
a. 0.20 M HNO3 and 0.40 M NaNO3
YES
NO
b. 0.20 M HNO3 and 0.40 M HF
YES
NO
c. 0.20 M HNO3 and 0.40 M NaF
YES
NO
d. 0.20 M HNO3 and 0.40 M NaOH
YES
NO
e. 0.10 M KOH and 0.20 M CH3NH3Cl
YES
NO
6
5. (16 pt) Lactic acid is found in muscles after exercising and has a Ka = 1.4 x 10-4.
i. You have a 250.0-mL sample of 0.800 M lactic acid, assuming no volume change, what mass
of NaOH must be added to make the best buffer?
a. 1.0 g
b. 2.0 g
c. 3.0 g
d. 4.0 g
e. none of these
ii. What is the pH of a 350.0 mL solution that contains 0.060 moles lactic acid and 0.040 moles
sodium lactate?
a. 4.03
b. 3.67
c. 3.85
d. 2.63
e. 2.45
iii. What is the pH of the solution described in part ii after the addition of 0.015 moles of HNO3?
a. 3.94
b. 4.33
c. 3.36
d. 4.28
e. 3.43
iv. What is the pH of the solution described in part ii after the addition of 0.015 moles of KOH?
a. 3.94
b. 3.67
c. 3.25
d. 4.46
e. 3.36
7
6. (8 pt) Calculate the percentage of pyridine (C5H5N) that forms pyridinium ion, C5H5NH+, in a
0.10 M aqueous solution of pyridine (Kb = 1.7 x 10-9)
8
7. (10 pt) Calculate the Ka for an unknown monoprotic acid HX, given that a solution of 0.10 M
LiX has a pH of 8.90.
9
8. (10 pt) Isocyanic acid (HNCO) can be prepared by heating sodium cyanate in the presence of
solid oxalic acid according to the equation: 2 NaOCN (s) + H2C2O4 (s) → 2 HNCO (l) + Na2C2O4 (s)
Upon isolating pure HNCO (l), an aqueous solution can be prepared by dissolving the liquid
HNCO in water. What is the pH of a 100.0 mL solution of HNCO prepared from the reaction of
10.0 g each of NaOCN and H2C2O4, assuming all of the HNCO produced is dissolved in
solution? (Ka of HNCO = 1.2 x 10-4.)
10
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
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)
Quadratic Equation: x =
− b ± b 2 − 4ac
2a
pH = pKa + log ([A-]/[HA])
Kw = 1 x 10-14 at 25oC
11