Exam 3
CH 201
PRACTICE EXAM
Name: ______________________________________
(please print)
I agree to have this graded exam returned in class
where others may see my grade :
Student I D : __________________
_____________________________________
(signature)
Instructions - Read Carefully
1.
Please show your work, and put your final answers in the spaces provided.
2.
Point values for each question are given in parentheses at the beginning of each question
HClO : 7.46
HBrO : 8.70
NH41+ : 9.25
H2S : 7.00
HS1- : 12.89
H2C2O4 : 1.23
HC2O41- : 4.19 HCl : < 0
HClO4 : < 0
H3BO3 : 9.27
HN3 : 4.72
H3PO4 : 2.21
pKa’s HCN : 9.40
1A
1
H
HNO3 : < 0
HNO2 : 3.40
HBr : < 0
H2PO41- : 7.20
HPO42- : 12.4
8A
2
He
3
Li
2A
4
Be
3A
5
B
4A
6
C
5A
7
N
6A
8
O
7A
9
F
6.941
9.012
10.81
12.01
14.01
16.00
19.00
20.18
11
Na
12
Mg
5B
6B
7B
15
P
16
S
17
Cl
18
Ar
24.31
4B
14
Si
22.99
3B
13
Al
19
K
20
Ca
21
Sc
22
Ti
23
V
24
Cr
25
Mn
26
Fe
27
Co
39.10
40.08
44.96
47.88
50.94
52.00
54.94
55.85
37
Rb
38
Sr
39
Y
40
Zr
41
Nb
42
Mo
43
Tc
85.47
87.62
88.91
91.22
92.91
95.94
55
Cs
56
Ba
57
La
72
Hf
73
Ta
74
W
132.9
137.3
138.9
178.5
181.0
183.8
87
Fr
88
Ra
89
Ac
104 105 106
Rf Db Sg
(223)
226.0
227.0
(261)
(262)
(263)
(264)
(265)
(268)
(269)
(272)
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
63
Eu
64
Gd
65
Tb
140.1
140.9
144.2
(145)
150.4
152.0
157.3
90
Th
91
Pa
92
U
93
Np
94
Pu
95 96
Am Cm
232.0
231.0
238.0
237.0
(244)
(243)
1.008
8B
4.003
10
Ne
1B
2B
26.98
28.09
30.97
32.07
35.45
39.95
28
Ni
29
Cu
30
Zn
31
Ga
32
Ge
33
As
34
Se
35
Br
36
Kr
58.93
58.69
63.55
65.39
69.72
72.61
74.92
78.96
79.90
83.80
44
Ru
45
Rh
46
Pd
47
Ag
48
Cd
49
In
50
Sn
51
Sb
52
Te
53
I
54
Xe
(98)
101.1
102.9
106.4
107.9
112.4
114.8
118.7
121.8
127.6
126.9
131.3
75
Re
76
Os
77
Ir
78
Pt
79
Au
80
Hg
81
Tl
82
Pb
83
Bi
84
Po
85
At
86
Rn
186.2
190.2
192.2
195.1
197.0
200.6
204.4
207.2
209.0
(209)
(210)
(222)
107
Bh
108 109 110 111
Hs Mt Ds Rg
66
Dy
67
Ho
68
Er
69
Tm
70
Yb
71
Lu
158.9
162.5
164.9
167.3
168.9
173.0
175.0
97
Bk
98
Cf
99
Es
100 101 102 103
Fm Md No Lr
(247)
(251)
(252)
(257)
(247)
(258)
(259)
Pledge: I have neither given nor received help during this exam.
Signature: ______________________________
Name (print): ___________________________
(262)
1) (20) How many grams of NaBrO (Mm = 118.9 g/mol) must be added to 750 mL of a 0.5 M
solution of HBrO in order to prepare a pH = 8.9 buffer?
Solution: The pKa of HBrO is 8.7. Use Hendersen-Hasselbach to solve for BrO- concentration.
g
2) (20) A buffer composed of HClO/NaClO solution contains 0.4 M of the acid and 0.4 M of the
conjugate base in a 1 liter solution..
A.) what is the pH of the buffer solution? B.) What concentration of HCl must be added to this
solution to maintain the buffer solution within one pH unit of the target value calculated in part
A. Assume that you are adding drops of concentrated HCl and so the volume does not change.
Solution: A.) Since the concentration of
the pH = pKa, of pH = 7.46
B.) HCl is an acid so the pH would move to one pH unit lower, or pH = 6.46.
We can do the calculation but we know from our discussion in class that this corresponds to a
ratio of
= 1/10. So we have
Therefore, one must add 0.327 M HCl.
pH of buffer =
concentration of HCl =
3) (10)A solution is labeled 0.250 M NaBr(aq). What is the concentration of HBr in this NaBr
solution?
Solution: zero.
[HBr] =
4) (20) What is the pH of a solution composed of 0.5 L of 0.7 M NaCN and 0.1 L of 0.1 M HCl ?
Solution: The final volume is 0.5 L + 0.1 L = 0.6 L.
Calculating the dilution factors we have
We can assume that the HCl is completely dissociated, and that it reacts with CN- to product
HCN.
Make a table
HCN
x
x
CN0.583
0.583-x
H+
0.0167
0.0167-x
We can neglect the term on the left hand side since Ka << 0.5997
Since x is essentially completely used up use H-H equation
pH =
5) (20) 50.0 mL of a 0.3 M H2PO41- solution is added to 50.0 mL of 0.04 M NaOH. What is the
pH of the resulting mixture?
We obtain Kb from Ka.
We check to see whether the OH completely converts the acid (
(
).
HPO420.0
x
x
However, since
H2PO410.150
-x
0.150-x
) into its conjugate base
OH10.02
-x
0.02-x
, we see that we can neglect Kb in this problem.
Alternative easier route is simply to assume that the base completely converts the acid into its
conjugate base (clearly justified by the above calculation):
pH =
6) (20) What is the pH (to nearest 0.1 pH unit) of a solution prepared by mixing 75.0 mL of 0.6
M HCl and 25.0 mL of 1.8 M NaOH?
Solution: The total volume is 75 mL + 25 mL = 100 mL
Initial [H+] = (0.6 M)(75/100) = 0.45 M
Initial [OH-] = (1.8 M)(25/100) = 0.45 M
[H+] - [OH-] = 0.45 M – 0.45 M = 0.0 M
pH =
7) (20) A.) Find the value of K for the equilibrium established when aqueous solutions of NH41+
and H3BO3 are mixed. B.) Calculate Go for this reaction at 298 K.
Solution: Write the balanced chemical equation:
This equation is composed of two acid equilibria
The equilibrium constant is
K=
8) (10) What are the hydronium and hydroxide ion concentrations in a pH = 2.7 solution?
Solution:
[H3O1+] =
M
[OH1-] =
M
9) (25) At 298 K in the stratosphere the partial pressures of O3 and O2 are 10-9 and 10-3 atm,
respectively. The formation of ozone is given by:
3 O2  2 O3
The free energy of formation of O3 is Go = +142.7 kJ/mol. Calculate A.) the equilibrium
constant and B.) G for the reaction in the stratosphere based on the actual pressures there.
NOTE: The reason why the reaction is not in equilibrium in the stratosphere is that the sun’s
radiation excites O2 and causes a bond breaking reaction to make O atoms that combine with O2
to form O3.
A.) The equilibrium constant is:
B.) The free energy, G is
K=
10) (25) At some temperature, K = 5 for the following reaction:
H2(g) + CO2(g)  H2O(g) + CO(g)
The initial partial pressures of H2 and CO2 are each 10 atm. The system is then allowed
to equilibrate. What is the equilibrium pressure of CO2?
Solution:
\
PCO2 = ____________________