CHEMISTRY 221 - Dr. Powers
Second Exam - FALL 2006
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SECOND EXAM
Lab Section______
October 26, 2006
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12
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Chemistry 221 – Second Exam Fall 2006
page 2
Name
Question No. Possible Points Points Earned TA Initials
1
20
2
5
3
6
4
12
5
32
6
10
7
15
Total
100
Chemistry 221 – Second Exam Fall 2006
Some Useful Information:
page 3
Name
Chemistry 221 – Second Exam Fall 2006
page 4
Name
USEFUL EQUATIONS
Monoprotic Weak Acid:

[H ] 
 Ka 
K a2  4 ( 1 )( F )( K a )
2
Isoionic point:
[H  ] 
K 1 K 2 F  K1 K w
K1  F
Debye-Hϋckel Equation:
 0.51 z 2 
log  




1   
305 


Diprotic Weak acid:
[H  ] 
K 2 K1 F  K1 K w
K1  F
Triprotic Weak acid:
[H  ] 
K 2 K1 F  K1 K w
K1  F
Gas Constant:
8.314472 J/K.mol
[H  ] 
K2 K3 F  K2 K w
K2  F
Chemistry 221 – Second Exam Fall 2006
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Name
1. (20 points) Identify which reactions are expected to proceed spontaneously and which are
expected to be non-spontaneous. Please explain why.
a. (8 points) Conversion of n-hexane to 2,2-dimethylbutane at room temperature (25oC)?
compound
Ho (kcal.mol) So (cal/mol.K)
n-hexane
-167
+389
2,2-dimethylbutane
-186
+359
b. (4 points) Conversion of n-hexane to 2,2-dimethylbutane during the petroleum cracking
process (400oC)?
c. (4 points) Ionization of acetic acid in water (Ka = 1.75x10-5)?
d. (4 points) Ionization of HCl in water?
Chemistry 221 – Second Exam Fall 2006
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2. (5 points) Based strictly on your answers to problem one (ignore your vast knowledge of
chemistry), which of the above reactions are expected to proceed rapidly?
3. (6 points) The charge state of amino acids play a significant role in how enzymes catalyze a
chemical reaction. Histidine, serine and aspartic acid are amino acids commonly found in enzyme
active-sites and form a “catalytic triad”. The backbone -carboxylic acid and -amino group are
involved in a peptide bond and pKa values are no longer relevant. The pKa value for the aspartic
acid side-chain is 3.86 and the histidine side-chain pKa value is 6.10.
a. (4 points) What is the protonation state of aspartic acid and histidine at pH 7.0?
O

OH
HO
pH 7.0
 NH2
O
aspartic acid
O

N
OH
HN
pH 7.0
 NH2
histidine
b. (2 points) What happens if the pKa of histidine is shifted to 7.2?
Chemistry 221 – Second Exam Fall 2006
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4. (12 points) Provide an example for the following terms. Please write out a complete chemical
reaction using real compounds to illustrate the concepts (do not use a general description).
a. Lewis acid
b. Protic Acid
c. Brønsted-Lowry Base
d. Base/Conjugate acid pair
Chemistry 221 – Second Exam Fall 2006
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5. (32 points) What is the Hg2+2 concentration when Hg2Cl2 (Ksp = 1.2x10-18) is dissolved in:
a. (6 points) pure water
b. (8 points) 0.01 M NaOH
c. (10 points) 0.01M NaCl.
Chemistry 221 – Second Exam Fall 2006
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d. (4 points) Please explain why the Hg2+2 concentrations differ?
e. (4 points) What is the charge balance and mass balance equations for c?
Chemistry 221 – Second Exam Fall 2006
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6. (10 points) Fe3+ and Cu2+ form complexes with EDTA (Cu2+-EDTA: Kf = 5×1018, Fe2+-EDTA: Kf
=1.7×1024). Fe3+, EDTA, and the Fe3+-EDTA complex are colorless. The Cu2+-EDTA complex is
colored a deep blue. The following spectrophotometric titration curve was obtained when a
solution containing both Fe3+ and Cu2+ was titrated with EDTA and absorbance was measured at
745 nm (red).
B
Absorbance
A
a. (2 points) What happens at point A?
b. (2 points) What happens at point B?
c. (3 points) A new technician is being trained using Fe3+ and Cu2+ standards. He is using a
calibrated buret and obtains a similar titration curve as above, but his measured
concentrations are systematically high. Why?
d. (3 points) Describe one method that would correct the technician’s mistake.
Chemistry 221 – Second Exam Fall 2006
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7. (15 points) A solution is composed of 0.1 M acetic acid (pKa = 4.756) in H2O (ignore activity
coefficients).
Acetic acid
acetate
a. (5 points) What is the pH of this solution?
b. (5 points) What is required to make the above solution a buffer with a pH of 3.0?
c. (2 points) Is this an optimal pH for this buffer?
Chemistry 221 – Second Exam Fall 2006
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d. (3 points) If instead of acetic acid, a polyprotic acid such as citric acid was used with pKa
values of 3.13, 4.76 and 6.40 how would you make a buffer with a pH of 3.0? (No need to
do the math, just provide a brief explanation)