Name:
Grading key
page 1/10
Chemistry 125/126, Exam 1
Tuesday, November 9, 2010
Welcome to the first hourly exam for Chemistry 125/126. This exam contains … (page 10).
A few notes about the exam. To receive credit … show your calculations; you will not receive credit for
numerical answers alone…. The exam should take about l hour, however, you may use up to 1.5 hours to
complete the exam. Save any questions … GSIs have been told not to answer questions during the exam.
Your graded exam will be available starting tomorrow. If, after checking the exam key, you feel there was a
mistake in grading, please turn your exam in to Prof. Kerner (mailbox 1500o chem.) with a statement
describing the mistake. Regrades must be turned in by noon, Monday, Nov. 22, 2010.
Course Information Section
111
113
115
117
119
121
123
125
127
129
131
133
135
137
139
141
143
145
147
149
GSI
M AM Hung Ki Yoon
M AM Kristen Ko
M AM Mallory Van Dongen
M AM Deidra Gerlach
M AM Lauren Soblosky
M AM Christina Franzese
M AM Shuwen Sun
T AM
Wendi Hale
T AM
Erik Guetschow
T AM Neranga Abeyasinghe
T AM
Yuwei Liu
T AM
Shuwen Sun
T AM
Molly Soper
T AM
Shuai Niu
T PM Mahmoud El Azzouny
T PM
Charity Haynes
T PM
Ken Fletcher
T PM
Ying Zhou
T PM
Wencheng Ge
T PM
Ping Guo
Section
GSI
151
211
213
215
217
219
221
223
225
227
229
231
233
235
237
239
241
243
245
247
Shi Jin
Wen Wen
Akiko Kochi
Kevin Hartman
Deidra Gerlach
Russell Bornschein
Jeanne Hankett
Joshua Skodack
Wendi Hale
Andy Crawford
Ken Fletcher
Yuwei Liu
Lauren Soblosky
Molly Soper
Ben Coupland
Hung Ki Yoon
Erik Guetsc how
Mallory Van Dongen
Shi Jin
Russell Bornschein
TPM
W AM
W AM
W AM
W AM
W AM
W AM
W AM
W PM
W PM
W PM
W PM
W PM
W PM
W PM
Th AM
Th AM
Th AM
Th AM
Th AM
Section
249
251
311
313
315
317
319
321
323
325
327
329
333
335
339
341
343
345
347
349
GSI
Th AM Christina Franzese
Th AM Ben Coupland
Th PM Mahmoud El Azzouny
Th PM Akiko Kochi
Th PM Catherine Mocny
Th PM Neranga Abeyasinghe
Th PM Wencheng Ge
Th PM Ping Guo
Th PM Shuai Niu
F AM
Wen Wen
F AM
Charity Haynes
F AM
Catherine Mocny
F AM
Deidra Gerlach
F AM
Joshua Skodack
F AM
Kristin Ko
F AM
Andy Crawford
F AM
Kevin Hartman
F AM
Russell Borenschein
F AM
Ying Zhou
F AM
Jeanne Hankett
For use by grader
Page
Points Score
15
Name:
2
GSI:
3
Section:
4
06
07
14
5
e-mail:
11
14
6
10
7
12
8
Page 1/10
Bonus
(+03)
Total
75 (+03)
For use by grader
Points out of
Name:
Grading key
page 2/10
Question 1 (15 points) asks you to make predictions based on your knowledge of periodic trends and
the relationship of structure to properties and reactivity. Periodic tables are provided at the back of this
exam for your reference.
A. Predict the comparative solubility in water of LiCl, CdCl2 and HgCl2.
Most soluble
least soluble
>
LiCl
>
CdCl2
HgCl2
3 points for correct order or zero
B.
Predict the relative oxidizing agent strength of the metal ions K+, Zn2+, and Ca2+.
Strongest
weakest oxidizing agent
>
Zn2+
Ca2+
>
K+
3 points for correct order or zero
C. You individually add the solid chloride salts of strontium ( Sr2+ ), chromium III ( Cr3+ ), and tin
II ( Sn2+ ), to water. Predict if the resulting solution is colored or colorless.
Circle any salt that is colored in aqueous solution.
CrCl3
SrCl2
SnCl2
+ 3 points for circling CrCl3; zero points if any other salt circled. No negative points.
D.
Correctly complete (circle your choice) each sentence: (+ 2 points for each correct choice)
#1. Bromine, Br, is more electronegative than (
Ni
Cl
#2. Salts of Sr2+ are likely to be more water soluble than salts of (
containing the same anion.
#3. The halide (
At -
Cl -
).
Ba2+
Ca2+
) is a better reducing agent than Br -.
For use by grader only
6 points
Page 2/10
For use by grader
Points out of
)
Name:
Grading key
page 3/10
Question 2 (6 points) deals with your preparation of solutions of cobalt chloride
A. How many grams of CoCl2 • 6H2O do you weigh out to prepare 200 mL of 0.10M cobalt (II)
chloride?
CoCl2 + 6H2O = 58.93 + 2 (35.5) + 6(2+16) = 237.93 g/mol (1 point for correct formula)
weight; work must be shown)
*237.93g
1 mol
x
0.10mol
1000mL
x 200mL = 4.76 grams to weigh out. ( *1 pt. for correct set-up)
*no double penalty – i.e. set-up ok if formula wt is same as calculated above.
( 1 point for correct answer)
grams of CoCl2 • 6H2O = 4.76
3 points
B. How many mmol of Cl-(aq) are in the resulting solution?
200mL x 0.10M CoCl2 = mmol CoCl2 ( 1 point for correct process)
= 20 mmol CoCl2 ( 1 point for correct calculation and reference to
CoCl2. For example if answer indicates 20 mmol Cl- that is incorrect)
2 x 20mmol = 40 mmol Cl- (1 point for recognizing stoichiometry)
mmol Cl- = 40 mmol
3 points
Page 3/10
For use by grader
Points out of
Name:
Grading key
page 4/10
Question 3. (18 points) involves using a spectrophotometer to determine the concentration of a
transition ion in aqueous solution (symbolized Trans Ion). The graphs below show the ions' absorption
spectrum (at a specific but undesignated concentration) and calibration graph at λ = 600nm).
Absorption Spectrum Trans Ion
1.2
Absorption
1
0.8
0.6
0.4
0.2
0
400
500
600
700
Wavelength (nm)
A. At what concentration (M) of Trans ion was the absorption spectrum obtained?
Circle the Molarity of trans ion used to produce the absorption spectrum:
0.15
0.2
0.32
0.5
1.25
3 points or zero
B. Your teammates dilutes the sample of Trans ion used to produce the above absorption spectrum.
She adds 9.00 mL of deionized water to 3.00 mL of the trans ion sample and generates another
absorption spectrum of the diluted sample from λ 400 to λ 700. Indicate if each statement below
is correct or not regarding the spectrum for the diluted sample.
Statement
1. The absorption spectrum for the diluted
sample will have absorption values = 1/3
the values shown in the above spectrum.
2. The absorptivity coefficient, Ε, increases
from λ 400 to λ 450, in the absorption
spectrum for the diluted sample.
Correct
Not Correct
X
X
2 points or zero each
Page 4/10
For use by grader
Points out of
total = 4 points
Name:
Grading key
page 5/10
C. What is the slope of the above calibration graph line (λ=600 nm) in units of absorbance/M?
Slope may be determined using Beer’s law or∆y/ ∆x
∆y/ ∆x = 1.25 – 0 abs/ 0.5 – 0 M (or 1.0 – 0 abs/ 0.4 – 0 M)etc. = 0.25 abs/M
- 1 point if inappropriate points are used to determine slope. If calculations are not shown not credit given
slope =
2.5 abs/M
(values 2.4 – 2.6 abs.M ok)
2 points
D. You produce a second calibration graph for Trans Ion at λ=450 nm. You use the same
samples, sample holder, and spectrophotometer. Will the slope of the resulting calibration
graph line be more, less, or the same as the above calibration line at λ=600 nm?
Indicate (circle) if the slope of calibration graph line at λ=450 will be
more than
less than
the same as
the slope at λ=600
3 points
E. Use the Beer-Lambert Law and the above calibration graph to determine the unknown concentration
(M) of a solution of Trans Ion that has an absorbance of 0.30 at λ = 600 nm. Show all calculations for
credit. You will not get credit for numerical answers alone.
Absλ 600 = Elc
0.30abs/M = 2.5 M (2 pts. for correct set-up; no double penalty if slope copied from
3B answer is wrong - – i.e. slope of 2.5 or slope value calculated under 3B is ok.
0.12M = c (1 pt. for correct calc. using slope from 3B)
Trans Ion M = 0.12 M
(0.11 – 0.13M ok)
3 points
F. You determine that a diluted sample of Trans Ion has a concentration = 0.25 M. You diluted the
sample by adding 5.0 mL of water to 1.0 mL of Trans Ion solution. What is the concentration (M) of
the original undiluted solution of Trans Ion? Show your calculations below.
Mi x Vi = Mf x Vf
Mi x 1.0mL = 0.25 M x 6.0mL (1 pt for correct set up and 1 pt. for correct values)
Mi = 0.25M x 6.0mL
1.0mL
Mi = 1.5 M
(1 pt. for correct calculation)
Trans Ion M = 1.5 M
Page 5/10
For use by grader
Points out of
Name:
Grading key
page 6/10
3 points
Question 4 (18 points) deals with your teams’ analysis of the reaction between 10 mL of
0.10M copper (II) bromide and 10 mL of 0.10M rubidium iodide:
CuBr2 (aq) + RbI (aq) → precipitate and color change
A.
You hypothesize that "Cu2+ is critical to formation of the precipitate and the color change".
1. What test reaction, by itself, could you use to directly prove if the hypothesis is true or false?
Record the specific reagents you will combine to test the hypothesis:
NaBr
+ RbI
1 pt for removing Cu2+ from CuBr2. There is no credit for the entire question if
another ion is removed or the reagent formula is invalid.
1 pt for substituting an acceptable soluble species for Cu2+ (acceptable instead of
CuBr2 are alkali chloride salts-- i.e. KBr or LiBr or RbBr...)
1pt. for reagent stoichiometry (i.e. – 1 pt if incorrect (e.g. KBr2 etc)
3 points
2. If your hypothesis is true what will you observe?
Expected observations if the hypothesis is true:
The products “precipitate AND color change” (no credit w/o reference to
both) will NOT form
or You observe no reaction occurring
3 points or zero
B.
You add different solvents to portions of the product mixture. Based on your studies of salt
solubility in different solvents, indicate if the amount of precipitate will decrease, increase, or
remain the same upon adding hexane or acetone.
1. Upon adding hexane to a sample of the product mixture, the amount of precipitate,
increases
remains the same
decreases
2 points or zero
2. Upon adding acetone to a sample of the product mixture, the amount of precipitate,
increases
decreases
remains the same
2 points
C.
You believe that copper (I) bromide or copper (II) iodide may be the precipitate. Indicate
(circle) if each copper compound is white or a color other than white.
Compound
White or other color? (Circle)
CuBr
White
CuI2
White
other color
other color
2 points or zero each
Page 6/10
For use by grader
Points out of
4 points
Name:
D.
Grading key
page 7/10
The Table below lists some species according to actual oxidizing agent strength.
Oxidizing agent strength:
Br2 > I2 > Cu+ > Cu2+ > Rb+
Based solely on the above table, identify (circle) the equation below correctly describing
the expected redox behavior of the copper species in a mixture of CuBr2(aq) and RbI(aq).
a)
Cu2+ + 2I- → I2 + Cu
c)
Cu2+ + I- → no reaction
b)
Cu2+ + 2I- → I2 + 2Cu+
d)
2Cu + I2 → 2I- + 2Cu+
4 point or zero
Question 5 (6 points) This question asks you to make predictions regarding redox reactions involving
metals and metal ions (0.10 M chlorides) in aqueous given the reducing agent strength:
Ni > Fe2+ > Pd
A. Complete the table below to indicate if a reaction occurs (YES) or does not occur (NO).
Ni
Fe2+
NO
Pd
NO
NiCl2
YES
NO
FeCl3
YES
YES
PdCl2
3 points
0.5 pt for each correct observation (i.e yes or no) for total of 3 points
B. What are the relative oxidizing agent strengths of Ni2+, Fe3+, and Pd2+?
Strongest oxidizing agent
Pd2+
Weakest oxidizing agent
>
Fe3+
>
Ni2+
3 points or zero
Page 7/10
For use by grader
Points out of
Name:
Grading key
page 8/10
Question 6 (12 points) deals with a study of potential redox reactions involving the metal ions thallium
(Tl) and iron (Fe) and halides where the observations below were made. Note: Hexane was added to all
product mixtures . Common oxidation states of the cations are: Tl+ and Tl3+ and Fe2+ and Fe3+.
1. FeCl3 (aq)
+ NaBr(aq)
yellow-brown
colorless
→ Product?
aqueous layer is yellow- brown; hexane layer is colorless.
2. TlCl3 (aq)
colorless
→ Product?
aqueous layer is colorless; hexane layer is yellow-orange.
+ NaBr(aq)
colorless
You carry out some further tests:
3. FeCl2 (aq)
+ Br2 (aq)
→ Product?
yellow-green
yellow-orange aqueous layer is yellow-brown; hexane layer is colorless
4. NaCl (aq)
colorless
+ NaBr (aq) → Product?
colorless
aqueous layer is colorless; hexane layer is colorless.
A. Based on test reaction 4, what can you conclude about reaction 2?
What can you conclude about the TlCl3(aq) + NaBr(aq) reaction from test 4?
Tl3+ (+2 for correct species and charge; -1 if charge on Tl incorrect)
is a reactant (or an Oxidizing agent) (+1)
and critical for production of the yellow-orange hexane layer (+1)
(or must be causing oxidation of Br- to Br2 etc)
4 points
B. Analyze test reaction 3 to complete the table below. Do not record spectators for product species.
Reaction
3
Oxidizing Agent
Reducing Agent
Net Reaction Product species
FeCl2
Br2
(or Fe2+)
2 point
2 point
Br-
Fe3+
(both species must be correct
= 2 pts) or FeBr3.
Wrong/no credit if answer includes
spectator species
2 points
C. From reactions 2 and 3 together, you can determine the relative strengths of Tl3+ vs. Fe3+
as oxidizing agents. Complete the table below.
Oxidizing agent strength of Tl3+ vs. Fe3+:
Strongest
Tl3+
Weakest Oxidizing Agent
>
Fe3+
2 points
Page 8/10
For use by grader
Points out of
Name:
Grading key
page 9/10
BONUS QUESTION (3 points or zero)
The bonus questions deals with the products of reaction 2 (question 6):
TlCl3 (aq)
colorless
+ NaBr (aq)
colorless
→
Products?
colorless aqueous layer contains a white precipitate;
hexane layer is yellow-orange.
Identify the white precipitate based on the following test observations:
1.Tl(NO3)3(aq)
+ NaBr(aq)
→
colorless aqueous layer contains a white precipitate;
hexane layer is yellow-orange.
2.TlCl3(aq)
+ NaNO3 (aq)
→
colorless aqueous layer; hexane layer is colorless.
Formula of the white precipitate from the TlCl3(aq) + NaBr(aq) reaction?
TlBr
3 points or zero
Page 9/10
For use by grader
Points out of