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Chemistry 125/126, Exam 2
Tuesday, April 23, 2013
Welcome to the second hourly exam for Chemistry 125/126. This exam consists of 8 questions worth
a total of 75 points plus a bonus question worth 3 points for a possible total of 78 points. It is 8 pages
long; 6 pages of questions plus one page with periodic tables (page 8).
To receive credit, your answers must be placed in the indicated spaces or boxes. If calculations are
required to obtain an answer, show your calculations; you will not receive credit for numerical
answers alone. Do not write in any box marked "For use by grader".
The exam should take about l hour, however, you may use up to 1.5 hours to complete the exam. All
exams must be turned in at 7:45 pm. Save any questions you have during the exam for Prof. Kerner.
The GSIs have been told not to answer questions during the exam.
Graded exams will be available starting tomorrow. If, after checking the exam key, you feel there
was a grading mistake, turn your exam in with a statement describing the mistake to Prof. Kerner’s
mailbox in 1500o chem. All re-grades must be turned in by noon, Monday, April 29, 2013.
Course Information
Section
GSI
127
129
135
137
139
T AM
T AM
T PM1
T PM1
T PM1
Daniel Bae
Bing Fu
Peter Eckert
Kaitlynn Wilson
Daniel Bae
Section
GSI
195 T PM2
Jiawei Xing
199
225
227
231
Section
GSI
237 Th PM2
T PM2 Non Ngernsutivorakul 239 Th PM2
ThPM1 Oluwasegun Adegoke
241 Th PM2
ThPM1 Kaitlynn Wilson
251 F PM1
Mallory
vanDongenTh PM1
253 F PM1
Sohmer
141 T PM1 Oluwasegun Adegoke 233 Th PM1 Peter Eckert
143 T PM2 Yaoxin Li
235 Th PM2 Phillip McClory
191 T PM2 Phillip McClory
119 F PM2
123 F PM2
Jiawei Xing
Non Ngernsutivorakul
Yaoxin Li
Kimberly Daley
Mallory vanDongenSohmer
Kimberly Daley
Bing Fu
For use by grader
Page
Name:
2
3
GSI:
4
5
Section:
6
7
(Bonus)
Total
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Points
15
10
12
12
15
11
(+03)
75 (+03)
Score
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Question 1 (9 points) asks you to make pH predictions based on your knowledge of properties and
structure. Periodic tables are provided at the back of this exam for your reference.
A. Predict the comparative pH of 0.10 M solutions of Mg(OH)2, Al(OH)3, and KOH.
Lowest pH
highest pH
<
<
3 points
B. Predict the comparative pH of 0.10 M solutions of Pb(NO3)2, Mg(NO3)2, and Ba(NO3)2.
Lowest pH
highest pH
<
<
3 points
C. Predict the comparative pH of 0.10 M solutions of Na3PO4, Na2HPO4, and NaH2PO4.
Lowest pH
highest pH
<
<
3 points
Question 2 (6 points) asks you to experimentally distinguish between pairs of solid white salt
compounds based on their Lewis Acid-Base behavior. The tests that you can perform are:
A. Determine the pH values of aqueous solutions of the compounds.
B. Determine if the compounds dissolve in 5 M NH3.
C. Determine if the compounds dissolve in 5 M NaOH.
Circle the one letter (A, B, or C) that corresponds to the one best test for distinguishing between the
two compounds given below.
Compound
Ag2CO3
vs
Compound
CaCO3
Test
KNO3
vs
Pb(NO3)2
A
B
C
PbC2O4
vs
CaC2O4
A
B
C
A
B
C
For use by grader only 6 points
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Question 3 (10 points) deals with the titration of 0.165g of a carboxylic acid with 0.10 M NaOH
where you determine that the unknown’s equivalent weight = 59.14
A. Based on the equivalent weight data, how many mL of 0.10 M NaOH did your team use to reach
the equivalence point? Record your calculations and answer.
mL of 0.10 M NaOH =
3 points
B. Based solely on the equivalent weight, and assuming an uncertainty of 5%, list any compound
from the table below that may be the unknown.
Compound
Formula
MW
1. Acetic acid
CH3COOH
60
2. Acrylic acid
CH2=CHCOOH
72
3. Maleic acid
HOOCCH=CHCOOH
116
4. Adipic acid
HOOCCH2CH2CH2CH2 COOH
146
Based solely on the equivalent weight the unknown may be:
4 points
C.
You weigh out another 0.165g of the carboxylic acid and record the mass prior to repeating
the titration with 0.10 M NaOH. Without realizing it, you lose 0.023g of the weighed sample
while transferring it to the titration flask. Will your determined equivalent weight be greater
than, equal to, or less than the prior determined equivalent weight of 59.14?
Correctly complete (circle your choice) the statement describing the effect of the error.
The equivalent weight will be (
greater than
the prior determined equivalent weight of 59.14.
3 points
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equal to
less than
)
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Question 4 (6 points) deals with the pH values of acids and bases below:
Sample
pH
Sample
pH
0.010 M HClO4
2.00
0.010 M HCOOH
2.95
0.010 M HC2H3O2
3.40
0.010 M NaOH
12.0
A.
Based on the above pH data, indicate the comparative pH of 0.10 M solutions of NaClO4,
NaC2H3O2, and NaCOOH.
Highest pH
lowest pH
______________
> _____________________ > _______________
3 points
B.
Based on the above pH data, identify (circle) the BEST indicator choice for the titration of
HClO4 with NaOH.
Indicator pH end point
Indicator pH end point
methyl red (pH 4.8-6.0)
p-naphtholbenzene (pH 8.2-10.0)
phenol red (pH 6.6-8.0)
alizarine yellow (pH 10.1-12.0)
3 points
Question 5 (6 points) deals with your investigation of the reaction:
5 mL 0.10 M CaI2 (aq) + 5 mL 0.10 M PdCl2 (aq) → formation of a solid in a brown solution
Colorless
red
Information: Pd2+ exists as [Pd(H2O)4]2+ in aqueous solution
Hypothesis:
"This is a Lewis acid-base precipitation reaction where the solid product is [Pd(H2O)2(I)2]".
Which, if any, of the test observations below supports the hypothesis? Check (√ ) the "YES"
column if the observation supports or “NO” column if the observation does not support the hypothesis.
Test
Observation
1. Add 5 mL of 0.10 M Ca(NO3)2 to
1. No reaction.
5 mL of 0.10 M PdCl2.
2. Two phases form. The upper
2. Add 5 mL of hexane to the
hexane phase is pink in color.
reaction product mixture.
3. Add 5 mL of 5 M NH3 to the
reaction product mixture.
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3. No reaction.
YES
NO
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Question 6 (12 points) deals with reactions of 0.10 M CuCl2, 0.10 M Na2CO3, and 1.0 M NH3.
Reaction Observations:
1. CuCl2(aq)
+
Na2CO3(aq) → blue ppt. forms
Clear blue
colorless
2.
NH3(aq)
→ blue ppt dissolves; solution turns violet-blue.
colorless
Information: Reactions 1 and 2 are Lewis acid-base reactions involving [Cu(H2O)4]2+.
A.
Mixture from step 1 +
Complete and balance the net Lewis acid-base equation for reaction 1:
____[Cu(H2O)4]2+ +
___ CO32- =
________________________________
3 points
B.
Record the identity of the species in your completed net Lewis acid-base equation (A) that
reacts with the added NH3(aq) (step 2 above) :
_______________________________________________
3 points
C.
What will you visually observe when you conduct the following test:
Reaction 2 product mixture (violet-blue solution)
+ 1 M HNO3
→ ___________ ?
Visual observations of the test results:
2 points
D.
What will you observe if you reverse the order of addition of reagents to the 0.1 M CuCl2?
Reactions:
1. CuCl2(aq)
Clear blue
Visual observations?
+
NH3(aq)
colorless
→
2. Step D1 mixture + Na2CO3(aq) →
colorless
_____________________________________
_____________________________________
4 points
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Question 7 (20 points) deals with your analysis of the reaction:
FeCl3(aq)
+
CrBr2(aq)
→
change of color
+
violet precipitate
where you hypothesize that the products are Fe2+, Cl - , Cr2+, and Br2.
A. Assume your hypothesis (above) is correct and complete the following statements.
1. Addition of hexane to the reaction mixture will yield two phases where the hexane phase is:
(circle your choice):
(
colorless
yellow-green
yellow-orange
).
2. Reacting species changing oxidation state are: __________________________ .
(Be sure you indicate the appropriate symbol and charge!)
3. Fe3+ is a better oxidizing agent than:
(circle your choice)
(
Cl2
Cl-
Br-
Br2
).
9 points
B-C.
In the boxes below tell what you learn, if anything, about the reaction of FeCl3(aq) and
CrBr2(aq) from each experimental test result given below (B-C). (For complete credit, make
sure you indicate the appropriate symbol and charge on any species you refer to.)
B. FeCl3(aq) +
Cr(NO3)2(aq) → change of color + violet precipitate
(same properties as products of reaction under analysis)
Test B demonstrates that:
3 points
C. Fe(NO3)3(aq) +
CrBr2(aq) → change of color + no precipitate
(same color change as reaction under analysis)
Test C demonstrates that:
3 points
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D.
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You conduct some additional tests to understand the reaction of FeCl3(aq) and CrBr2(aq):
NaCl(aq) +
CrBr2(aq) → no reaction
FeCl3(aq) +
NaBr(aq) → no reaction
1. Based on tests B-D is the original hypothesis correct? Indicate (circle) your answer.
Yes
No
2 points
2. Based on tests B-D record the identity (symbol and charge) of any spectator species in
the FeCl3(aq) + CrBr2(aq) reaction.
3 points
Question 8 (6 points) deals with “analysis of a reaction” where you add Pb(s) to 0.10 M FeCl2(aq)
and hypothesize that no reaction is occurring.
Circle any observation (1, 2, and/or 3) that can help confirm that NO reaction took place.
Observation
1.
Add hexane to the reaction mixture and observe that the hexane
remains clear and colorless.
2.
Add Pb(s) to 0.10 M FeCl3 and observe no reaction.
3.
Add Fe(s) to 0.10 M PbCl2(aq) and observe a reaction.
BONUS question (3 points) refers to question 6 and the
Reaction Observations:
Step 1. CuCl2(aq)
+
Clear blue
Step 2.
Na2CO3(aq) → blue ppt. forms
colorless
Mixture from step 1 + NH3(aq)
Colorless
→ blue ppt dissolves; solution turns violet-blue.
Upon evaporation of the step 2 product mixture, a solid violet-blue salt appears.
What is the formula of the violet-blue salt? _____________________________
3 points
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