CHEM 108 (Fall-2003)
Exam 2 (100 pts)
Name: -------------------------------------------------------------------------------, SSN -------------------------------LAST NAME, First
(Circle the alphabet segment of your LAST NAME): A-C
D-H
I-L
M-S
T-Z
Please answer the following questions:
Part I: Multiple Choices (52 pts: 13 @ 4 pts each). Circle the ONE best answer:
1. Assuming the reaction below is at equilibrium, which of the following changes will drive the
reaction to the right (product side)?
CO2 ( g)
C (s) + O2 (g)
ΔH˚ = -394 kJ
1. In creasing temperature
2. Removing CO2 (g)
3. Adding C (s)
a) 1 only
b) 2 only
c) 3 only
d) 2 and 3
2. Rank the following 0.1 M solutions in order of increasing pH (lowest pH =1, highest = 4):
NaBr, KNO2, KOH and NH4Cl
a) KNO2 < KOH < NaBr < NH4Cl
b) NH4Cl < NaBr < KNO2 < KOH
c) KOH < KNO2 < NaBr < NH4Cl
d) NH4Cl < KNO2 < KOH < NaBr
3. Given the following Ka values:
HOCl
2.9 x 10-8; HClO3
5.0 x 102; HCN
6.2 x 10-10 ; HF
6.6 x 10-4
Which of the following is the strongest base?
a) CN-
b) ClO3-
c) OCl-
d) F-
4. At the equivalence point of the reaction of HNO3 (aq) and aqueous NH3
a) pH < 7
b) pH > 7
c) pH = 7
d) pH depends on the Ka and Kb values.
5. What is the relationship between the pKa for an acid base indicator and the pH at which the
indicator changes color?
a) pH = pKa
b) pH = Ka
c) pH = - log [pKa]
d) [H+] = pKa
6. What is the conjugate acid of ammonia is
a) H+
b) NH4+
c) NH2-
d) NH3
7. What is the solubility of AgBr (Ksp = 5.2 x 10-13, molar mass = 187.7 g/mol) in moles per liter?
a) 7.1 x 10-7
b) 5.1 X 10-13
c) 1.3 x 10-4
d) 5.1 x 10-5
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8. Calculate the equilibrium constant for the following reaction of a weak acid and a base
HClO (aq) + OH- (aq)
ClO- (aq) + H2O (aq)
Given Ka (HClO) = 2.8 x 10-8 and Kw = 1.0 x 10-14
a) 2.8 x 106
b) 3.6 x 10-7
c) 2.8 x 10-22
d) no answer was given
\
9. The solubility product expression, Ksp for Ca3(PO4)2
{Ca3(PO4)2 (s)
3 Ca2+ (aq) + 2 PO43- (aq)} is
a) Ksp = [Ca2+]3[PO43-]2/[Ca3(PO4)2]
b) Ksp = [Ca2+][PO43-]
c) Ksp = [Ca2+]3[PO43-]2
d) Ksp = [3 Ca2+]3[2 PO43-]2
10. Which o the following reactants, when mixed in equal mole quantities, forms a buffer?
a) HF and NaF
b) NH4+ and HCl
c) HCl and NaCl
d) NaC2H3O2 (sodium acetate) and NaOH
11. A buffer may be prepared by mixing a weak acid with a roughly equivalent amount of strong
base. Which of the acids below I best for the preparation of a buffer with a pH of 9.00?
a) chlorous acid, HClO; Ka = 2.8 x 10-8
b) formic acid, HCO2H; Ka = 1.9 x 10-4
c) dihydrogen phosphate ion, H2PO4-, Ka = 6.2 x 10-8
d) ammonium ion, NH4+; Ka = 5.6 x 10-10
12. If 10.0 g of NaOH (40.01 g/mol) is added to 500.0 mL of 0.520 M HNO2, will the mixture be:
a) basic
b) acidic
c) neutral
d) amphoteric
13. Which of the following sulfides has the greater molar solubility?
CdS (Ksp = 8 x 10-25), CoS (Ksp = 8 x 10-22), CuS (Ksp = 6 x 10-37), ZnS (Ksp = 2 x 10-25)
a) Cd S
b) CoS
c) CuS
d) ZnS
____________________________________________________________________________________
Part II (48 pts + 6 pts bonus) Calculations: Show all work for full credit. Please express all
answers with proper units and correct number of significant figures.
1. (12 pts) Consider the following system at 25°C where Kp = 0.11:
N2O4 (g)
2 NO2 (g)
ΔH˚ = +57.2 kJ
a) Predict the direction (right, left, no effect) in which the reaction will occur to reach the
equilibrium when 0.20 mol of N2O4 and 0.20 mol of NO2 are introduced into a 4.0 L container.
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b) Predict the direction if the reaction was carried out at a temperature higher than 25°C.
c) What is the effect of decreasing the pressure on the direction of the reaction?
2. (12 pts) What is the pH of a solution obtained by adding 1.15 mg of aniline hydrochloride,
C6H5NH3+Cl- (a weak organic acid with Ka = 1.4 x 10-5, molar mass = 129.6 g/mol) to 3.18 L of
0.105 M aniline, C6H5NH2
pH =
3. (8 pts) Calculate the solubility of PbI2 (Ksp = 7.1 x 10-9, molar mass = 461 g/mol) in grams/mol, in
presence of 0.10 M of KI solution?
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4. (16 pts + 6 pts bonus for part e) In the titration of 25.0 mL of 0.175 M benzoic acid, HBz (Ka =
6.6 x 10-5) with 0.269 M KOH solution:
a)
Write a balanced net ionic equation for the reaction at the equivalence point
.
b) What volume of potassium hydroxide is required to reach the equivalence point?
c) Calculate the pH of the solution before the titration begins?
d) What is the pH at the mid point of the titration?
e) What is the pH at the equivalence point?
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PREFERENCE SHEET FOR
CHEM 108
Exam 2 – Fall 2003
You will have 50 minutes to complete this exam.
The exam has 4 pages plus Periodic Table and Reference page.
When you are told to do so, tear off the Periodic Table cover sheet and use as required during
exam.
Useful Information:
R = 0.0821 L atm/mol.K = 8.31 J/K.mol
K = 273 + C
Useful Equations:
d (density) = mass (m)/volume (V)
π = MRT
ΔT = i. K.m
Rate = k[A]
ln [A]o = kt
[A]
2
Rate = k[A]
1/[A] - 1/[A]o = kt
1 atm = 760 torr = 760 mmHg
PV = nRT
Psolv = χsolv.Posolv.
ln[A] = - kt + ln[A]o
Rate = k
[A] = - kt + [A]o
k = Ae-Ea/RT
ln k = (-Ea/R)(1/T) + ln A
t1/2 = ln 2/k = 0.693/k
t1/2 = 1/k[A]o
t1/2 = [A]o/2k
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1
2
H
Periodic Table of the Elements
He
1.01
3
2
3
4.00
4
5
6
7
8
9
10
Li Be
B C N O F
6.94 9.01
10.81 12.01 14.01 16.00 19.00 20.18
11 12
13 14 15 16 17 18
Na Mg
Al Si P
22.99 24.30
26.98 28.08 30.97 32.06 35.45 39.95
S
Ne
Cl Ar
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
4
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
39.1 40.08 44.96 47.88 50.94 52.00 54.94 55.85 58.93 58.69 63.55 65.38 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
5
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I
Xe
85.47 87.62 88.91 91.22 92.91 95.94 (98) 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.1
55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
6
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
132.9 137.3 138.9 178.5 181.0 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222)
7
87 88 89 104 105 106 107 108 109 110 111 112
Fr Ra Ac Rf Db Sg Bh Hs Mt Uun Uuu Uub
(223) 226.0 227.0 (261) (262) (263) (262) (265) (266) (269) (272) (277)
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
140.1 140.9 144.2 (145) 150.4 152.0 157.2 158.9 162.5 164.9 167.3 168.9 173.0 175.0
90 91 92 93 94 95 96 97 98 99 100 101 102 103
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) (260)
5
6
7