Chem 1721/1821
Exam 3
April 24, 2014
ID#: 2013172 _____
Dr. Susan E. Bates
Name ____________________________________
Please do not remove this page. The periodic table, constants, and
equations you may need are on the last page of this exam. Feel free to
remove it, and use it as scrap paper. All pages must be turned in.
Be sure to fill out and bubble in the proper letters and numbers on the
ScanTron sheet for the following information:
your name
Test Form A
the 9-digit ID number given above (rightmost digit blank)
You should answer questions for Part II (1 - 15) on the ScanTron sheet.
You will not have the ScanTron returned to you, so if you would like to know
what you answered after the exam is returned, circle your answers on this form
as well.
Chem 1721/1821: Exam 3
Name _____________________________________
Part I (40 points): Short Answer.
1.
Sodium bromite (NaBrO2) is dissolved in water. What will be the pH of the resulting solution? Circle the one best
answer below.
pH = 7.0
2.
Ksp = 108x5
Ksp = 256x4
Ksp = 27x4
pH < 7.0
Ksp = 4x3
pH > 7.0
Which one of the following solutions should have the highest [OH–]? Circle the one best answer.
For HNO2, pKa = 3.35; for HF, pKa = 3.17.
0.40 M NaCl (aq)
5.
Ksp = 9x4
In the titration of a weak base with a strong acid, what will be true of the pH of the solution at the stoichiometric point?
Circle the one best answer below.
pH = 7.0
4.
pH > 7.0
A salt is composed of M3+ cation and Y– anion. What is the mathematical relationship between Ksp and molar
solubility (x) for this salt? Circle the one best answer below.
Ksp = x2
3.
pH < 7.0
0.40 M KNO2 (aq)
0.40 M CaF2 (aq)
Determine the molar concentration of hydroxide ion that is required to cause precipitation of Cu(OH)2 (s) from a
solution containing 1.2 x 10–4 M Cu2+ (aq). For Cu(OH)2, Ksp = 2.6 x 10–19.
[OH–] = ___________________ M
6.
True or False? The percent ionization of a weak acid, HA is smaller in the presence of a strong acid than in pure water.
__________________
7.
Ammonium nitrate dissolves readily in water at 25°C. As the salt dissolves, the solution becomes quite cold.
a.
What is the sign of ∆H° for this process?
_____________________
b.
What is the sign of ∆S° for this process?
_____________________
c.
Which of the following is true of ∆G° for this process? Circle the one best answer below.
∆G° is – at all T's
8.
∆G° is + at all T's
∆G° is – at low T's
∆G° is + at low T's
a.
Write the net ionic equation that describes the reaction that occurs when HCl (aq) is added to a buffer solution
containing NaHSO3 (aq) and Na2SO3 (aq). Include all physical states and charges on ions.
b.
As a result of the reaction that occurs when HCl (aq) to the buffer described in a above, indicate whether each of
the following quantities in the buffer solution will increase (I), decrease (D), or remain constant (RC).
[HSO3–] _________________
[SO32–] __________________
[H3O+] _________________
9.
Ni(OH)2 (s)  Ni2+ (aq) + 2 OH– (aq); Ksp = 2.0 x 10–15
Consider the following equations:
Ni2+ (aq) + 4 CN– (aq)  [Ni(CN)4]2– (aq); Kf = 1.0 x 1031
a.
Write a balanced chemical equation (including physical states and charges on ions) that describes the overall
reaction that takes place when a saturated solution of nickel (II) hydroxide is treated with cyanide ion.
b.
Determine the value of the equilibrium constant, K, for the reaction described in a above. KC = ________________
c.
True or False? The solubility of Ni(OH)2 in 0.25 M KCN (aq) is greater than the solubility in 0.25 M NiCl2 (aq).
________________
10.
Bromocresol purple is an acid/base indicator. In its acidic form it is yellow, and in its basic form it is purple. If a small
amount of bromocresol purple is added to a 0.50 M NH4Cl (aq), what color will the solution be? Circle the one best
answer below.
yellow
11.
green
purple
colorless
Consider a buffer solution with pH = 9.78 that is composed of 0.15 M HCN (aq) and 0.45 M KCN (aq).
Determine Ka for HCN.
Ka = _______________________
12.
Consider the figure to the right depicting a system composed of 3 particles in a
container segmented into 3 chambers.
a.
Which system (in the microstate shown) has the greater entropy? Circle the
one best answer below.
system i
b.
/ 40
system ii
both have the same entropy
Which transformation(s) would occur with with a negative value of ∆Ssys? Circle the one best answer below.
system i  system ii
system i  system ii and system ii  system i
system ii  system i
neither system i  system ii or system ii  system i
Part II (45 points): Multiple Choice. Indicate on the ScanTron sheet provided the one best answer for each of the
following.
1.
Consider that a solution of HClO4 (aq) is titrated with a solution of NaOH (aq). Which one of the following statements
about this titration is false?
a. At the stoichiometric point in this titration, mol HClO4 = mol NaOH.
b. A buffer solution exists beyond the stoichiometric point in this titration.
c. Before the stoichiometric point OH– is the limiting reactant.
d. An appropriate indicator for this titration would change color at a pH very close to 7.0.
e. The "pH vs. volume NaOH added" curve for this titration starts, initially at very low pH, and ends at very high pH.
2.
Consider the following reaction:
C2H4 (g) + 3 O2 (g)  2 CO2 (g) + 2 H2O (g); ∆S° = –29.4 J/K
S° (J/K•mol): ?
205.0
213.7
188.7
Calculate S° for C2H4 (g) given the standard molar entropy values and ∆S° for the reaction.
3.
a. –21.8 J/K•mol
b. –1390 J/K•mol
d. 219.2 J/K•mol
e. 228.8 J/K•mol
The weak acid H2PO4– has Ka = 6.2 x 10–8. Determine the value of pKb for HPO42–.
a. 7.42
4.
5.
c. –160.4 J/K•mol
b. 7.21
c. 7.00
a. 5.13
b. 5.42
d. 8.58
e. 8.87
c. 7.84
Which one of the following will result in increased solubility of PbBr2?
b. adding KBr (aq)
d. none of these will increase solubility
7.
9.
c. adding Pb(NO3)2 (aq)
e. all of these will increase solubility
25.0 mL of a 0.789 M solution of a weak acid (HA, pKa = 5.45) is titrated with 0.434 M KOH (aq). What volume of the
base must be added to achieve a solution with pH = 5.45?
a. 25.0 mL
b. 12.5 mL
c. 22.7 mL
d. 8.56 mL
c. 45.4 mL
In your Solubility of Calcium Iodate experiment, hydorxynaphthol blue was used as an indicator. What is the color of
the soution when a significant concentration of Ca2+ in the solution is bound to this indicator?
a. pink
8.
e. 6.58
Determine the pH of a saturated solution of Zn(OH)2 at 25°C. For Zn(OH)2, Ksp = 2.1 x 10–16.
a. adding HNO3 (aq)
6.
d. 6.79
b. colorless
c. blue
d. green
e. yellow
Benzoic acid, HC7H5O2, is a weak acid with Ka = 6.5 x 10–5 at 20°C. Determine ∆G° for the acid ionization of HC7H5O2
at 20°C
a. –220 kJ/mol
b. 16 J/mol
d. 230 J/mol
e. 23 kJ/mol
c. 1.6 kJ/mol
Determine the pH of the solution that results when 20.0 mL of 0.100 M benzoic acid (pKa = 4.19) is mixed with 12.0
mL of 0.050 M NaOH (aq).
a. 4.19
b. 4.39
d. 3.82
e. 4.54
c. 3.94
10.
11.
The molar solubility of cadmium fluoride is 2.0 x 10–4 mol/L. Calculate Ksp for CdF2.
a. 8.0 x 10–12
b. 3.2 x 10–11
d. 8.0 x 10–8
e. 4.0 x 10–4
c. 4.0 x 10–8
Which one of the following statments is false?
a. The melting of ice at 0°C proceeds with an increase in entropy that is greater than the increase in entropy that occurs
as solid ice is heated to increase its temperature from –10°C to –1°C.
b. The decomposition of liquid water to O2 and H2 proceeds with an increase in entropy.
c. All spontaneous processes are exothermic.
d. For a spontaneous reaction, the Gibb's free energy decreases as the reaction proceeds.
e. When a system is at equilibrium, the Gibb's free energy does not change.
12.
Consider the following balanced redox reaction:
5 H2SO3 + 2 MnO4–  2 Mn2+ + 5 SO42– + 4 H+ + 3 H2O
Which statement below is true for this reaction?
a. H undergoes a 1 e– oxidation, and MnO4– is the reducing agent.
b. Mn undergoes a 5 e– oxidation, and MnO4– is the oxidizing agent.
c. O is both oxidized and reduced in this reaction.
d. S undergoes a 4 e– reduction, and H2SO3 is the oxidizing agent.
e. S undergoes a 2 e– oxidation, and MnO4– is the oxidizing agent.
13.
Consider 1.0 M solutions of the following salts: NaNO2, (CH3NH3)Cl, Ca(NO2)2, KCl, and Sr(ClO4)2. Which one of the
following correctly puts these solutions in order of increasing pH?
a. Ca(NO2)2 < NaNO2 < Sr(ClO4)2 = KCl < (CH3NH3)Cl
b. (CH3NH3)Cl < KCl = Sr(ClO4)2 < NaNO2 < Ca(NO2)2
c. KCl < NaNO2 < (CH3NH3)Cl < Ca(NO2)2 < Sr(ClO4)2
d. (CH3NH3)Cl < NaNO2 < Ca(NO2)2 < KCl < Sr(ClO4)2
e. Sr(ClO4)2 < KCl < NaNO2 < Ca(NO2)2 < (CH3NH3)Cl
14.
Which of the following would be the best choice as the acid component of a buffer solution with pH = 4.60?
a. H3AsO4 (pKa = 2.19)
b. H2C6O6 (pKa = 4.18)
d. CH3NH3+ (for CH3NH2 pKb = 3.36)
15.
c. HC3H5O3 (pKa = 3.87)
e. C6H5NH3+ (for C6H5NH2 pKb = 9.38)
In experiment 24 you use absorbance measurements to determine pKa of an acid-base indicator. In this analysis a
student made a plot of log (AHIN–A/A–AIn–) vs. pH. The data were fit to a straight line, and the equation for that line is
y = 0.998x – 3.7. What is the pKa of this indicator?
a. 0.998
b. –0.998
c. 3.7
d. –3.7
e. 2.70
/ 45
Part III (15 points): Problems. Work all of the problems on the next page. Show all work; work with units in all steps;
report all answers with the appropriate units and significant figures. No partial credit will be given if I cannot follow your
work.
1.
Magnesium arsenate, Mg3(AsO4)2, is a sparingly soluble salt with Ksp = 2.0 x 10–20 at 25°C.
a.
Write the balanced chemical equation that descirbes the solubility equilibrium for this salt. Include all physical
states and charges on ions.
b.
Determine the solubility (in g/L) of Mg3(AsO4)2 in water at 25°C. For Mg3(AsO4)2, molar mass = 350.8 g/mol.
_________________________ g/L
2.
Consider the following reaction for the formation of methanol:
CO (g) + 2 H2 (g)  CH3OH (g); ∆H° = –90.8 kJ and ∆S° = –219.2 J/K.
a.
Determine the value of ∆G° (in kJ) for this reaction at 25°C.
∆G° = ________________________ kJ
b.
Determine the value of ∆G (in kJ) at 25°C when PCO = 2.25 atm, PH2 = 1.50 atm, and PCH3OH = 0.0500 atm.
∆G = ________________________ kJ
3.
Acrylic acid (HC3H3O2) is a weak acid with Ka = 5.5 x 10–5. A buffer solution is prepared by combining equal volumes
of 0.140 M HC3H3O2 (aq) and 0.185 M NaC3H3O2 (aq).
a.
Use the Henderson-Hasselbalch equation to determine the pH of this buffer solution.
pH = _____________________
b.
Determine the pH of this buffer solution after the addition of 0.0200 mol KOH to 750 mL of the buffer. Assume
no volume change after the addition of the KOH.
pH = _____________________
/ 15
Some information that you may find useful:
pKa + pKb = pKw
pKw = 14.00 at 25°C
pX = –logX
pH + pOH = pKw
ΔG° = ΔH° – TΔS° = –RTlnK
IA
II A
III B
IV B
VB
pH = pKa + log([base]/[acid])
R = 8.314 J•K–1•mol–1
ΔG = ΔG° + RTlnQ
VI B
VIII
VII B
ΔG° = ΣnΔG° f (prod) – ΣnΔG° f (react)
IB
II B
III A
IV A
VA
VI A
1
VII A inert
gases
2
H
He
1.008
3
4
Li
Be
6.941 9.012
11
12
K
Ca
6
7
8
9
B
C
N
O
F
Ne
10.81 12.01 14.01 16.00 19.00 20.17
13
14
15
16
17
18
Na Mg
22.98 24.31
19
20
5
4.003
10
Al
21
22
23
24
Sc
Ti
V
Cr Mn Fe
25
26
27
28
Co
Ni
29
30
Si
P
S
Cl
Ar
26.98 28.09 30.97 32.06 35.45 39.95
31
32
33
34
35
36
Cu Zn Ga Ge
As
Se
Br
Kr
39.10 40.08 44.96 47.90 50.94 52.00 54.94 55.85 58.93 58.71 63.55 65.37 69.72 72.59 74.92 78.96 79.90 83.80
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Rb
Sr
Y
Zr
Nb Mo Tc
Ru Rh Pd Ag Cd
In
Sn Sb
Te
I
Xe
85.46 87.62 88.91 91.22 92.91 95.94 98.91 101.0 102.9 106.4 107.8 112.4 114.8 118.6 121.8 127.6 126.9 131.3
55
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
*
Cs
Ba
La
Hf
Ta
W
Re Os
Ir
Pt
Au Hg
Tl
Pb
Bi
Po
At
Rn
132.9 137.3 138.9 178.4 180.9 183.8 186.2 190.2 192.2 195.0 196.9 200.5 204.3 207.2 208.9 (210)
87
88
89
104
105
106
107
108
109
110
111
112
113
114
115
116
*
(210) (222)
117
118
(223) 226.0 (227)
Fr
Ra
Ac
Rf
Db Sg Bh Hs
(267)
(268)
58
59
Ce
Pr
(271) (272)
60
61
Mt
(276)
(269)
62
63
64
65
Gd
Tb
Nd Pm Sm Eu
140.1 140.9 144.2 (147) 150.4 151.9
90
91
92
93
94
95
Th
Pa
U
Ds Rg Cn Uut
(270)
(280)
(242)
(243)
66
67
Dy Ho
Fl Uup Lv Uus Uuo
(289)
(288)
(293)
(294)
68
69
70
71
Er Tm Yb
Lu
157.3 158.9 162.5 164.9 167.3 168.9 173.0 174.9
96
97
98
99
100
101
102
103
Np Pu Am Cm Bk
232.0 (231) 238.0 (237)
(285) (286)
(247)
(247)
Cf
(251)
Es Fm Md No
(254)
(253)
(256) (254)
Lr
(262)
(294)