CHM 146 Advanced Chemistry II
Name
SOLUTIONS
Exam 5, Spring 2010 – Dr. Steel
1. Circle any and all of the combinations in the group below that would be appropriate
choices to use to make a buffer. (6 points)
oxalic acid (Ka = 6.0×10-2) and potassium hydrogen oxalate
sodium dihydrogen citrate (Ka = 1.7×10-5) and sodium hydrogen citrate
potassium cyanide and hydrocyanic acid
hydrochloric acid and sodium chloride
carbonic acid and sodium sulfite
aniline (Kb = 3.9×10-10) and anilium chloride
2. For each of the combinations listed below, circle the acidic component and then
indicate the approximate buffer capacity (i.e, a range of pH) of a buffer solution
created from the combination. (6 points)
4.2 – 6.2
pyridine (Kb = 1.7×10-9) and pyridium chloride
(C6H5NHCl)
(C6H5N)
6.2 – 8.2
sodium sulfite and potassium hydrogen sulfite
(Na2SO3)
(KHSO3)
9.3 – 11.3
potassium hydrogen carbonate and potassium carbonate
(K2CO3)
(KHCO3)
3. Based on your Ka chart, what acid would you choose to make a buffer solution with a
pH of 9.34? (2 points)
The acid whose pKa is closest to this is hydrocyanic acid (9.40),
but boric acid, B(OH)3(H2O), could also be used (pKa = 9.14)
4. Based on their Kb values, which base in the group below would you choose to make a
buffer solution with a pH of 9.34? (2 points)
methylamine
Kb = 4.4×10-4
ammonia
Kb = 1.8×10-5
pKa of NH4+ = 9.25
pyridine
Kb = 1.7×10-9
aniline
Kb = 3.9×10-10
CHM 146 Advanced Chemistry II
SOLUTIONS
Name
Exam 5, Spring 2010 – Dr. Steel
5. Determine the pH of a solution prepared by mixing 30.0 mL of 0.184-M sodium
hypochlorite (NaClO) with 25.0 mL of 0.174-M hypochlorous acid. (10 points)
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6. What mass of pyridium chloride (C6H5NHCl) would need to be added to 250. mL of
aqueous 0.240-M pyridine (C6H5N, Kb = 1.7×10-9) in order to prepare a buffer whose
final pH is 5.72? (10 points)
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CHM 146 Advanced Chemistry II
Name
SOLUTIONS
Exam 5, Spring 2010 – Dr. Steel
7. The Ksp of silver phosphate, Ag3PO4, is 8.9×10-17 at 25 °C. Calculate and compare
the molar solubility of silver phosphate in pure water, in a 0.100-M solution of silver
nitrate (AgNO3), and in a 0.100-M solution of sodium phosphate (Na3PO4).
(24 points)
Pure Water:
Ag3PO4(s) ↔ 3 Ag+(aq) + PO43-(aq)
[ ]0
--0
0
Δ[]
-x
+ 3x
+x
[ ]eq
---
3x
x
.
.
.
0.100 M AgNO3:
Ag3PO4(s) ↔ 3 Ag+(aq) + PO43-(aq)
[ ]0
--0.100
0
Δ[]
-x
+ 3x
+x
[ ]eq
---
0.100
x
.
.
.
0.100 M Na3PO4:
Ag3PO4(s) ↔ 3 Ag+(aq) + PO43-(aq)
--0
0.100
[ ]0
Δ[]
-x
+ 3x
+x
[ ]eq
---
3x
0.100
.
.
.
.
.
Thus, the order of increasing solubility is
AgNO3 << Na3PO4 << Water
CHM 146 Advanced Chemistry II
SOLUTIONS
Name
Exam 5, Spring 2010 – Dr. Steel
8. The titration of 60.0 mL of 0.212 M quinine (Kb = 3.3×10-6) is carried out using
0.244 M hydrochloric acid.
a. Calculate the pH of the solution after the addition of 17.8 mL of acid. (10 points)
B
+
H3 O +
BH+
→
+
molo
0.0127
0.00434
0
Δ mol
- 0.00434
-0.00434
+0.00434
molfin
0.00838
---
0.00434
[ ]fin
0.108
---
0.0558
H2O
.
.
.
.
.
.
.
b. Determine the volume of acid you need to add to reach the equivalence point and
the pH that results at the equivalence point. (10 points)
At the equivalence point:
.
.
.
.
The moles of acid and base will react in equal ratio and produce a number of moles
of conjugate acid equal to the moles of base initially (0.0127 mol = 0.113 M):
BH+
+
H2 O ↔
B + H3 O +
[ ]o
0.113
0
0
Δ[]
-x
+x
+x
[ ]eq
0.113
x
x
.
.
.
.
c. Calculate the pH of the solution after the addition of 58.5 mL of acid. (10 points)
B
+
H3 O +
→
BH+
+
molo
0.0127
0.0143
0
Δ mol
- 0.0127
-0.0127
+0.0127
molfin
-----
0.0016
0.0127
0.0135
0.107
[ ]fin
.
H2O
CHM 146 Advanced Chemistry II
Name
SOLUTIONS
Exam 5, Spring 2010 – Dr. Steel
9. For each structure shown below, indicate what computational model you would use to
determine the molecular energy. Select from molecular mechanics (MM), quantum
mechanics (QM), or a hybrid approach (QMMM). (4 points)
OH
1-octen-3-ol
QM
Strand of Gal4:DNA
MM
10. Consider the conformational scan data plotted below for propane. The dihedral under
study is shown to the right of the data.
Dihedral Angle (°)
‐117.602
‐117.603 0
30
60
90
120
150
180
Energy (H)
‐117.604
‐117.605
‐117.606
‐117.607
‐117.608
‐117.609
a. At what dihedral angle(s) is propane most stable? (2 points)
60° and 180°
b. At what dihedral angle(s) are the hydrogen atoms on the carbon labeled #1 closest to
the hydrogen atom labeled #4? (2 points)
0° and 120°
c. Estimate the molecular energy for a dihedral angle of 270°. (2 points)
-117.606 H