Download FIRST HOURLY EXAMINATION

FIRST HOURLY EXAMINATION
CHEMISTRY E-1ax
October 2, 2014
6:00 pm – 8:00 pm
LAST NAME: ________________________________________
FIRST NAME: ________________________________________
ROW: ___________
SEAT: __________
SECTION TF: Joann Kim
NOTES:
1. This exam has 6 pages with 6 problems, plus the cover sheet and scrap paper. Please do not
separate any sheets of the exam. A separate sheet with the Useful Information and a Periodic
Table will be distributed; this sheet will not be collected and nothing on it will be graded.
2. Note the point values of each exam question, and consider where you can best spend your
time.
3. Answer all questions on the exam sheets. Put your final answers in the boxes provided. If
you must use the back, then please indicate that clearly!
4. When time is called, the exam is over and you must immediately stop writing.
5. For full credit, show all calculations; this also helps us award partial credit.
6. All numerical answers must include correct units for full credit.
1. ____________ / 16
2. ____________ / 16
3. ____________ / 16
4. ____________ / 18
5. ____________ / 18
6. ____________ / 16
TOTAL: ___________ / 100
1
1.
Mercury, Hg, is an industrial pollutant that can accumulate in fish and can cause birth
defects or developmental disorders if consumed by pregnant women or young children.
a)
The most common isotope of mercury is mercury-202. Determine the number of
protons (p), neutrons (n), and electrons (e) in one atom of mercury-202. (3 pts)
# of p =
b)
# of e =
Mercury accumulates in fish (and in humans) in the form of the polyatomic ion
methylmercury, which has the chemical formula CH3Hg+ and which bonds strongly to
sites in the amino acid cysteine. Determine the number of protons (p) and electrons (e) in
one ion of methylmercury, CH3Hg+. (2 pts)
# of p =
c)
# of n =
# of e =
A common salt containing the methylmercury cation is the salt methylmercury chloride,
which has the chemical formula CH3HgCl. Using average molar masses from the
periodic table, determine the molar mass of CH3HgCl and determine the percent by mass
of mercury in this salt. (4 pts)
molar mass of CH3HgCl =
% by mass of Hg in CH3HgCl =
d)
Tilefish from the Gulf of Mexico can have some of the highest levels of mercury, with
average levels of 1.50 mg of mercury atoms (Hg) per kg of fish. Determine the number of
mercury atoms contained in a 6.00 ounce serving of tilefish with these mercury levels. (7 pts)
number of Hg atoms =
1
g/mol
%
atoms
( ______ / 16 pts)
2
2.
Upon heating, solid calcium chlorate, Ca(ClO3)2, decomposes into solid calcium
chloride, CaCl2, and oxygen gas.
a)
Write a complete, balanced equation for the decomposition of calcium chlorate as
described above. (Please include state symbols such as (s), (aq), etc.) (4 pts)
b)
This is an oxidation-reduction reaction. Indicate which element is being oxidized and which
element is being reduced in this reaction. (circle one element in each box below) (2 pts)
Oxidized:
Ca
Cl
O
Reduced:
Ca
Cl
O
1.057 grams of calcium chlorate are heated, and this decomposition reaction occurs.
Unfortunately, however, the sample is not heated enough, and this reaction does not
proceed to completion. The total mass of the solids (calcium chloride product as well as
unreacted calcium chlorate reactant) when the reaction is stopped is 0.660 grams.
c)
Determine the mass of oxygen gas that was given off during this reaction. (2 pts)
mass of O2 =
d)
Determine the mass of calcium chloride formed in this reaction. (4 pts)
mass of CaCl2 =
e)
Determine the percent yield for this reaction. (4 pts)
percent yield =
2
( ______ / 16 pts)
3
3.
Compound X is an unknown solid that contains only the elements C, H, and O.
a)
When 10.00 grams of Compound X are completely combusted in excess oxygen gas,
18.33 g of CO2 and 5.00 g of H2O are collected. Determine the empirical formula of
Compound X. (6 pts)
empirical formula:
b)
When 10.00 grams of Compound X are titrated with 1.50-molar KOH, it is discovered that
Compound X is a triprotic acid, and 92.6 mL of 1.50-molar KOH are required to completely
neutralize the sample. Determine the molecular formula of Compound X. (6 pts)
molecular formula:
c)
3
Write a complete, balanced equation for the combustion of solid Compound X.
(Please include state symbols such as (s), (aq), etc.) (4 pts)
( ______ / 16 pts)
4
4.
a)
Write an acceptable chemical name for each of the following. (1 pt each)
K2Cr2O7
BrCl3
FeCl3
b)
Write the oxidation state of each atom in the box above the atom. (1 pt per box)
Cu2SO4
c)
For each of the following, write a complete balanced equation for the chemical reaction
that is described. Be sure to include state symbols such as (s), (aq), etc. (3 pts each)
i) Solid sodium metal reacts with liquid water in an oxidation-reduction reaction,
forming aqueous sodium hydroxide and hydrogen gas.
ii) Liquid carbon disulfide combusts in the presence of oxygen gas to form the gaseous
products carbon dioxide and sulfur dioxide.
iii) Aqueous solutions of potassium cyanide and zinc nitrate are mixed together, resulting in
a precipitation reaction.
iv) Aqueous solutions of barium hydroxide and phosphoric acid are mixed together,
resulting in a reaction that is both an acid/base neutralization reaction and a
precipitation reaction.
4
( ______ / 18 pts)
5
5.
The following two solutions are mixed together:
100.0 mL of 1.400-molar lead (II) nitrate, Pb(NO3)2
400.0 mL of 0.500-molar sodium fluoride, NaF
A white precipitate is formed in an exchange reaction.
a)
Write a complete, balanced equation, and then a net ionic equation for this chemical
reaction. (Please include state symbols such as (s), (aq), etc.) (6 pts)
Complete:
Net Ionic:
b)
Calculate the mass of the lead (II) fluoride precipitate that would be formed, assuming
the reaction goes to completion, and determine the molar concentrations of all ions in the
resulting solution. (Note: Assuming that the reaction goes to completion, some of the
concentrations may be effectively zero.) (12 pts)
mass of PbF2 formed =
[Pb2+] =
[Na+] =
5
[NO3–] =
[F–] =
( ______ / 18 pts)
6
6.
Bromine, Br2, and chlorine, Cl2, will both react with hydrogen gas to form the
corresponding strong monoprotic acids. You are given a mixture of bromine and chlorine,
and you react this mixture completely with excess hydrogen gas. The resulting mixture of
HBr and HCl is titrated with 2.00-molar sodium hydroxide, and 110.0 mL of 2.00 M
NaOH are required to reach the endpoint of the titration. After the titration, the resulting
solution is mixed with excess AgNO3, causing the complete precipitation of solid AgBr
and solid AgCl. A total of 34.21 grams of precipitate is formed. Determine the number of
moles of Br2 and Cl2 in the original mixture. (16 pts)
(Note the following molar masses: AgBr = 187.8 g/mol
and
AgCl = 143.3 g/mol)
number of moles of Br2 =
moles
number of moles of Cl2 =
moles
6
( ______ / 16 pts)
Scrap Paper
Nothing on this page will be graded unless you clearly indicate
on a specific problem that additional work is located here.
NAME:_____________________________
Useful Information
This page will NOT be collected after the exam.
We will NOT grade anything written on this page.
Avogadro’s Number = 6.02 × 1023
Density of water at 25°C = 1.00 g/mL
Unit Conversions:
1 mL = 1 cm3
1 foot = 12 inches
1 inch = 2.54 cm
1 kg = 2.20 pounds
1 pound = 16 ounces
27
Co
28
Ni
29
Cu
30
Zn
Fr
Cs
227.03
Ac
138.91
89
La
Y
88.91
57
(261)
Rf
178.49
104
Hf
Zr
91.22
72
47.88
40
Actinide series
Lanthanide series
Ra
Ba
Sr
44.96
39
231.04
Pa
232.04
Th
140.91
91
140.12
90
59
Pr
58
Ce
(263)
183.85
[106]
W
Mo
95.94
74
52.00
42
(262)
Ha
180.95
105
Ta
Nb
92.91
73
50.94
41
238.03
U
144.24
92
60
Nd
(262)
186.21
[107]
Re
Tc
(98)
75
54.94
43
237.05
Np
(145)
93
61
Pm
(265)
190.20
[108]
Os
Ru
101.07
76
55.85
44
(244)
Pu
150.36
94
62
Sm
(266)
192.22
[109]
Ir
Rh
102.91
77
58.93
45
(243)
Am
151.96
95
63
Eu
195.08
Pt
Pd
106.42
78
58.69
46
(247)
Cm
157.25
96
64
Gd
196.97
Au
Ag
107.87
79
63.55
47
(247)
Bk
158.93
97
65
Tb
200.59
Hg
Cd
112.41
80
65.39
48
(251)
Cf
162.50
98
66
Dy
204.38
Tl
In
114.82
81
69.72
49
Ga
(252)
Es
164.93
99
67
Ho
207.20
Pb
Sn
118.71
82
72.61
50
Ge
(257)
Fm
167.26
100
68
Er
208.98
Bi
Sb
121.76
83
74.92
51
As
(258)
Md
168.93
101
69
Tm
(209)
Po
Te
127.60
84
78.96
52
Se
(259)
No
173.04
102
70
Yb
(210)
At
I
126.91
85
79.90
53
Br
35.45
35
Cl
226.03
26
Fe
S
32.07
34
19.00
17
(223)
25
Mn
P
30.97
33
16.00
16
137.33
88
24
Cr
Si
28.09
32
14.01
15
132.91
87
V
23
Al
26.98
31
12.01
14
87.62
56
Rb
Ca
Mg
10.81
13
(260)
Lr
174.97
103
71
Lu
(222)
Rn
Xe
131.29
86
83.80
54
Kr
Ar
39.95
36
20.18
18
Ne
85.47
55
Ti
22
F
40.08
38
21
Sc
O
39.10
37
K
Na
Li
24.31
20
N
9
22.99
19
C
8
9.012
12
B
7
6.941
11
Be
6
4.003
10
5
4
1.008
3
2
He
H
1
PERIODIC TABLE OF THE ELEMENTS