2021 U.S. NATIONAL CHEMISTRY OLYMPIAD - American Chemical Society
2021 U.S. NATIONAL
CHEMISTRY OLYMPIAD
LOCAL SECTION EXAM
Prepared by the American Chemical Society Chemistry Olympiad Examinations Task Force
OLYMPIAD EXAMINATIONS TASK FORCE
Seth N. Brown, Chair, University of Notre Dame, Notre Dame, IN
James Ayers, Colorado Mesa University, Grand Junction, CO
Jerry Bell, Simmons University, Boston, MA (retired)
Mark DeCamp, University of Michigan, Dearborn, MI (retired)
James Dohm, Exxon-Mobil Corporation, Houston, TX
Xu Duan, Holton-Arms School, Bethesda, MD
Valerie Ferguson, Moore HS, Moore, OK
Julie Furstenau, Thomas B. Doherty HS, Colorado Springs, CO (retired)
Kimberly Gardner, United States Air Force Academy, CO
Paul Groves, South Pasadena HS, South Pasadena, CA (retired)
John Kotz, State University of New York, Oneonta, NY (retired)
Sarah Leite, Hopkins School, New Haven, CT
Michael A. Morgan, Francisco Bravo Medical Magnet HS, Los Angeles, CA
Jane Nagurney, Scranton Preparatory School, Scranton, PA (retired)
Sheila Nguyen, Claremont Graduate University, Claremont, CA
DIRECTIONS TO THE EXAMINER
This test is designed to be taken with an answer sheet on which the student records his or her responses. All answers are to be marked
on that sheet, not written in the booklet. Each student should be provided with an answer sheet and scratch paper, both of which must
be turned in with the test booklet at the end of the examination. Local Sections may use an answer sheet of their own choice.
The full examination consists of 60 multiple-choice questions representing a fairly wide range of difficulty. A periodic table and other
useful information are provided on page two of this exam booklet for student reference.
Only non-programmable calculators are to be used on the ACS Local Section exam. The use of a programmable calculator, cell
phone, or any other device that can access the internet or make copies or photographs during the exam is grounds for disqualification.
Suggested Time: 60 questions¡ª110 minutes
DIRECTIONS TO THE EXAMINEE
DO NOT TURN THE PAGE UNTIL DIRECTED TO DO SO.
This is a multiple-choice examination with four choices for each question. There is only one correct or best answer to each question.
When you select your choice, blacken the corresponding space on the answer sheet with your pencil. Make a heavy full mark, but no
stray marks. If you decide to change your answer, be certain to erase your original answer completely.
Property of ACS USNCO ¨C Not for use as USNCO Local Section Exam after March 31, 2021
Distributed by the American Chemical Society, 1155 16th Street, N.W., Washington, DC 20036
All rights reserved. Printed in U.S.A.
amount of substance
ampere
atmosphere
atomic mass unit
Avogadro constant
Celsius temperature
centi¨C prefix
coulomb
density
electromotive force
energy of activation
enthalpy
entropy
equilibrium constant
ABBREVIATIONS AND SYMBOLS
Faraday constant
F molar mass
free energy
G mole
frequency
¦Í Planck¡¯s constant
gas constant
R pressure
gram
g rate constant
hour
h reaction quotient
joule
J second
kelvin
K speed of light
kilo¨C prefix
k temperature, K
liter
L time
measure of pressure mm Hg vapor pressure
milli¨C prefix
m volt
molal
m volume
molar
M year
n
A
atm
u
NA
¡ãC
c
C
d
E
Ea
H
S
K
CONSTANTS
M
mol
h
P
k
Q
s
c
T
t
VP
V
V
y
R = 8.314 J mol¨C1 K¨C1
R = 0.08314 L bar mol¨C1 K¨C1
F = 96,500 C mol¨C1
F = 96,500 J V¨C1 mol¨C1
NA = 6.022 ¡Á 1023 mol¨C1
h = 6.626 ¡Á 10¨C34 J s
c = 2.998 ¡Á 108 m s¨C1
0 ¡ãC = 273.15 K
1 atm = 1.013 bar = 760 mm Hg
Specific heat capacity of H2O =
4.184 J g¨C1 K¨C1
EQUATIONS
=
E Eo ?
1
1A
1
H
1.008
3
Li
6.941
11
Na
RT
ln Q
nF
? ??H o ? ? 1 ?
=
ln K ?
? ? ? + constant
? R ?? T ?
PERIODIC TABLE OF THE ELEMENTS
2
2A
4
Be
13
3A
5
B
9.012
12
Mg
22.99
24.31
19
K
20
Ca
3
3B
21
Sc
4
4B
22
Ti
5
5B
23
V
6
6B
24
Cr
7
7B
25
Mn
8
8B
26
Fe
9
8B
27
Co
10
8B
28
Ni
45
Rh
46
Pd
39.10
40.08
44.96
47.88
50.94
52.00
54.94
55.85
58.93
85.47
87.62
88.91
91.22
92.91
95.95
(98)
101.1
132.9
137.3
138.9
178.5
180.9
183.8
186.2
(223)
(226)
(227)
(261)
(262)
(263)
(262)
37
Rb
55
Cs
87
Fr
38
Sr
56
Ba
88
Ra
39
Y
57
La
89
Ac
40
Zr
72
Hf
104
Rf
58
Ce
41
Nb
73
Ta
105
Db
59
Pr
42
Mo
74
W
106
Sg
60
Nd
43
Tc
75
Re
107
Bh
61
Pm
11
1B
29
Cu
12
2B
30
Zn
14
4A
6
C
15
5A
7
N
16
6A
8
O
17
7A
9
F
16.00
19.00
20.18
26.98
28.09
30.97
32.07
35.45
39.95
31
Ga
32
Ge
33
As
34
Se
35
Br
36
Kr
15
P
16
S
17
Cl
18
Ar
58.69
63.55
65.39
69.72
72.61
74.92
78.97
79.90
83.80
102.9
106.4
107.9
112.4
114.8
118.7
121.8
127.6
126.9
131.3
190.2
192.2
195.1
197.0
200.6
204.4
207.2
209.0
(209)
(210)
(222)
(265)
(266)
(281)
(272)
(285)
(286)
(289)
(289)
(293)
(294)
(294)
44
Ru
76
Os
108
Hs
62
Sm
77
Ir
109
Mt
63
Eu
78
Pt
110
Ds
64
Gd
47
Ag
79
Au
111
Rg
65
Tb
48
Cd
80
Hg
112
Cn
66
Dy
49
In
81
Tl
113
Nh
67
Ho
50
Sn
82
Pb
114
Fl
68
Er
51
Sb
83
Bi
115
Mc
69
Tm
52
Te
84
Po
116
Lv
70
Yb
53
I
85
At
117
Ts
150.4
152.0
157.3
158.9
162.5
164.9
167.3
168.9
173.0
175.0
232.0
231.0
238.0
(237)
(244)
(243)
(247)
(247)
(251)
(252)
(257)
(258)
(259)
(262)
94
Pu
95
Am
96
Cm
97
Bk
98
Cf
99
Es
100
Fm
101
Md
102
No
54
Xe
86
Rn
118
Og
71
Lu
(145)
93
Np
10
Ne
14.01
14
Si
144.2
92
U
4.003
12.01
13
Al
140.9
91
Pa
18
8A
2
He
10.81
140.1
90
Th
Page 2
? k2 ? Ea ? 1 1 ?
ln=
? ?
? ? ?
? k1 ? R ? T1 T2 ?
103
Lr
Property of ACS USNCO ¨C Not for use as USNCO Local Section Exam after March 31, 2021
DIRECTIONS
?
When you have selected your answer to each question, blacken the corresponding space on the answer sheet using a soft, #2
pencil. Make a heavy, full mark, but no stray marks. If you decide to change an answer, erase the unwanted mark very carefully.
?
There is only one correct answer to each question. Any questions for which more than one response has been blackened will not
be counted.
?
Your score is based solely on the number of questions you answer correctly. It is to your advantage to answer every question.
1. At 120 ¡ãC and 1 atm pressure, 1.00 L of methane, CH4,
reacts completely with excess oxygen to form carbon
dioxide and water. What volumes of the two products are
produced at this pressure and temperature?
(A) 1.00 L CO2 and 2.00 L H2O
(B) 1.00 L CO2 and 4.00 L H2O
(A) Ionization of the acid is markedly exothermic.
(C) 2.00 L CO2 and 2.00 L H2O
(B) The solute associates partially into dimers at lower
temperatures.
(D) 2.00 L CO2 and 4.00 L H2O
2. Polypropylene is made by polymerizing propene, C3H6
(M = 42.1). How many molecules of propene must be
polymerized to make 3.50 g polypropylene?
(A) 1.43 ¡Á 10
22
(C) 6.02 ¡Á 1023
6. The concentration of an aqueous solution of a
nonvolatile, monoprotic acid is measured first by freezing
point depression and then by boiling point elevation. The
solution is found to be 0.93 m by freezing point
depression and to be 0.82 m by boiling point elevation.
Which is the best explanation for this discrepancy?
(B) 5.01 ¡Á 10
22
(D) 2.11 ¡Á 1024
3. An organic compound contains only carbon, hydrogen,
nitrogen, and oxygen. It is 61.71% C, 4.03% H, and
16.00% N by mass. What is its empirical formula?
(A) C5H4NO
(B) C9H7N2O2
(C) C10H8N2O
(D) C11H8NO2
4. 100.0 mL of 0.500 M CaBr2 and 50.0 mL of 1.00 M
NaBr are mixed. What is the concentration of bromide
ion in the resulting solution?
(C) The volume of the solution is greater at higher
temperatures.
(D) The boiling point elevation constant for water is
smaller than its freezing point depression constant.
7. When substances are separated by fractional distillation,
which property is least typical of the substance that
distills first?
(A) Highest molar mass
(B) Weakest intermolecular forces
(C) Greatest vapor pressure
(D) Lowest boiling point
8. Which element is most abundant (by mass) in the Earth¡¯s
crust?
(A) 0.500 M
(B)
0.667 M
(A) Carbon
(B) Oxygen
(C) 0.750 M
(D)
1.00 M
(C) Magnesium
(D) Silicon
5. 1.00 g of hydrated potassium carbonate, K2CO3?nH2O,
is heated to 250 ¡ãC to give 0.836 g anhydrous K2CO3.
What is the value of n?
(A) 0.16
(B) 1.0
(C) 1.5
(D) 2.0
9. Which gas has the highest molar solubility in water at
25 ¡ãC and 1 atm?
(A) CO2
(B) NH3
(C) O2
(D) H2S
10. Which compound is colorless?
(A) NaMnO4
(B) CrAsO4
(C) RbIO4
(D) BaCrO4
Property of ACS USNCO ¨C Not for use as USNCO Local Section Exam after March 31, 2021
Page 3
11. Chlorine gas is bubbled into a colorless aqueous solution
of sodium iodide. Which is the best description of what
takes place?
(A) A precipitate of white NaCl forms.
(A) Hydrogen bonding
(B) A precipitate of metallic Na forms.
(B) Covalent bonding between carbon atoms
(C) The solution turns pale green as the chlorine
dissolves.
(C) Dipole-dipole forces
(D) The solution turns yellow-brown as iodide reacts
with the chlorine.
12. A student determines the number of moles of water in a
hydrated metal oxide by weighing a clean, dry crucible
and lid while the crucible is empty, then reweighing the
crucible and lid with a sample of the hydrate, heating the
crucible and lid with a Bunsen burner and then
reweighing the crucible and lid with the sample after
cooling to room temperature. Which error will result in
too high a value for the amount of water of hydration?
(A) The heating is conducted only once instead of the
three times recommended by the procedure.
(B) The lid is left off the crucible when it is weighed
with the hydrated oxide.
(C) The metal oxide reacts partially with oxygen in the
air, forming a compound in a higher oxidation state.
(D) Some of the heated oxide is spilled from the crucible
before it can be weighed.
13. The diagram is a microscopic view of a snapshot of a
substance at equilibrium, with the circles representing
molecules and the arrows the molecules¡¯ velocities.
What state of matter is depicted?
(A) Solid
(B) Liquid
(C) Gas
(D) Plasma
14. The vapor density of which fluorocarbon is 6.17 g L¨C1 at
23 ¡ãC and 1.00 atm?
(A) C3F6
Page 4
15. What is the principal intermolecular force that must be
overcome when n-hexane (C6H14, bp = 69 ¡ãC) is
vaporized?
(B) C3F8
(C) C4F6
(D) C4F8
(D) London dispersion forces
16. The melting point of water decreases with increasing
pressure. Which is the best explanation for this
observation?
(A) Liquid water is denser than solid water at 0 ¡ãC.
(B) Melting of ice is endothermic at 0 ¡ãC.
(C) The vapor pressure of liquid water is lower than the
vapor pressure of solid water at 0 ¡ãC.
(D) Solid and liquid water cannot coexist at equilibrium
at 0 ¡ãC at pressures different from 1 atm.
17. A cylinder containing a mixture of CO and CO2 has a
pressure of 2.00 atm at 93 ¡ãC (366 K). The cylinder is
then cooled to ¨C90 ¡ãC (183 K), where CO is still a gas but
CO2 is a solid with a vapor pressure of 0.25 atm. The
pressure in the cylinder at this temperature is 0.90 atm.
What is the mole fraction of CO2 in the cylinder?
(A) 0.10
(B) 0.28
(C) 0.35
(D) It cannot be determined from the information given.
18. A mineral containing only copper and oxygen adopts the
cubic unit cell shown below. What is the formula of this
mineral?
(A) Cu2O
(B) CuO
(C) Cu3O2
(D) Cu4O9
19. Burning 48.0 g of graphite in excess oxygen under
standard conditions releases 1574.0 kJ of heat. What is
?H¡ãf of CO2(g)?
(A) 1574 kJ mol-1
(B) ¨C1574 kJ mol-1
(C) ¨C394 kJ mol-1
(D) 32.8 kJ mol-1
Property of ACS USNCO ¨C Not for use as USNCO Local Section Exam after March 31, 2021
20. Two metal samples, labeled A and B, absorb the same
amount of heat. Sample A has a mass of 10.0 g, and its
temperature increases by 38 ¡ãC. Sample B has a mass of
20.0 g, and its temperature increases by 23 ¡ãC. Which
sample has the greater specific heat capacity?
(A) Sample A
(B) Sample B
(C) Both samples have the same specific heat capacity.
(D) It is impossible to determine from the information
given.
21. A reaction has ?S¡ã > 0 and ?H¡ã > 0. Which statement
about this reaction must be correct?
(A) If carried out in a well-insulated flask, the
temperature of the reaction mixture will decrease.
(B) It will occur spontaneously at 298 K and 1 atm
pressure.
(C) As the temperature is raised, Keq for this reaction
decreases.
(D) The reaction has more moles of products than it has
moles of reactants.
22. What is the boiling point of water in a pressure cooker
with a pressure of 2.00 atm? (The enthalpy of
vaporization of water is 40.7 kJ mol¨C1.)
(A) 101 ¡ãC
(B) 121 ¡ãC
(C) 141 ¡ãC
(D) 200 ¡ãC
23. The gas-phase bromination of propene has a standard
enthalpy of reaction of ¨C122.5 kJ mol-1.
C3H6(g) + Br2(g) ¡ú C3H6Br2(g)
?H¡ã = ¨C122.5 kJ mol-1
Substance
C3H6(g)
Br2(g)
?H¡ãf, kJ mol-1
20.4
30.9
What is the standard enthalpy of formation of
C3H6Br2(g)?
(A) ¨C71.2 kJ mol-1
(B) ¨C102.1 kJ mol-1
(C) ¨C142.9 kJ mol-1
(D) ¨C173.8 kJ mol-1
24. Titanium has a normal melting point of 1668 ¡ãC and a
molar enthalpy of fusion of 14.15 kJ mol-1. The standard
molar entropy of liquid titanium is 97.53 J mol-1 K-1 at
1668 ¡ãC. What is the standard molar entropy of solid
titanium at this temperature?
(A) 89.05 J mol-1 K-1
(B) 90.24 J mol-1 K-1
(C) 97.52 J mol-1 K-1
(D) 104.82 J mol-1 K-1
25. In the reaction
Cl2(g) + 3 F2(g) ¡ú 2 ClF3(g)
the rate of disappearance of F2(g) is 1.0 M s-1. What is
the rate of appearance of ClF3(g)?
(A) 0.33 M s-1
(B) 0.67 M s-1
(C) 1.0 M s-1
(D) 1.5 M s-1
26. When the rate of the reversible reaction A + B ? C is
studied under a certain set of conditions, it is found that
the rate of the forward reaction is kf[A]. What can be
concluded about the rate law for the reverse reaction
under these conditions?
(A) Rate = kr[C]
(B) Rate = kr[C]
[B]
(C) The rate law of the reverse reaction cannot be
determined from the information given.
(D) An error must have been made, since if the reaction
is reversible, the forward rate law must be
Rate = kf[A][B].
27. A substance A decomposes irreversibly to form B. A
plot of ln([A]) as a function of time from the beginning
of the reaction until A is 97% consumed is a straight line
with a negative slope. What is the reaction order in A?
(A) Zero order
(B) First order
(C) Second order
(D) Third order
28. Which statement about catalysis is correct?
(A) If a catalyst increases the forward rate of a reaction
by a factor of two, it must increase the rate of the
reverse reaction by a factor of two.
(B) If a catalyst increases the rate of formation of a
product by a factor of two, it must increase the rate
of formation of the mirror image of the product by a
factor of two.
(C) A catalyst must be in the same phase as the reactants
and products of the reaction.
(D) A catalyzed reaction must proceed by the same
mechanism as the uncatalyzed reaction, but with a
lower activation energy.
29. The isotope 226Ra has a half-life for radioactive decay of
1600 y. How long will it take the amount of 226Ra in a
sample of 226RaCl2 to decrease by 25%?
(A) 660 y
(B) 800 y
Property of ACS USNCO ¨C Not for use as USNCO Local Section Exam after March 31, 2021
(C) 1200 y
(D) 1600 y
Page 5
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