Chem 128, Exam III
April 25, 2002
Name
I. (41 points)
A. (4 points) Write the equilibrium equation and Ksp expression for Co2 S3 . Pay attention
to chemical state designations, charges and stoichiometric coefficients!
equilibrium equation:
Ksp expression:
B. (14 points) Ksp for PbCrO4 (M = 323.2 g/mol) at 25◦ C is 2 x 10−14 . 1.000 mg of
PbCrO4 is added to 1.0 L of H2 O at this temperature.
1. How many mg of the of the PbCrO4 dissolves?
2. How many mg of the of the PbCrO4 remains undissolved?
3. If 1.000 mg of PbCrO4 are added to 1.00 L of 0.100 M Na2 CrO4 at 25◦ C, how
many mg of the of the PbCrO4 dissolves?
4. A mixture containing 100.0 mg of PbCrO4 in 1.0 L of water is heated to 95◦ C. A
saturated solution is obtained, i.e., all the PbCrO4 just dissolves. What is Ksp for
PbCrO4 at 95◦ C?
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Chem 128, Exam III
April 25, 2002
Name
C. (14 points) Consider the equilibrium curve for AgCl shown on the right. Questions 1
– 5 refer to a solution at point A on the curve. Answer the questions below, using Y
(for true ), F (for false ), or MI (for more information required ) in the blanks provided.
1. The solution is saturated
and at equilibrium.
2. Addition of NaCl increases the
concentration of Cl− in solution.
3. Addition of NaCl increases the
concentration of Ag+ in solution.
4. Addition of Ag+ results in the
precipitation of AgCl.
5. Addition of solid NaNO3 to the
solution without change in volume
does not change [Ag+ ] or [Cl− ].
6. At equilibrium, [Ag+ ] is always equal
to [Cl− ].
7. The molar solubility s, of AgCl can be
expressed in terms of Ksp as Ksp = 2s2 .
D. (9 points) The addition of OH− to a saturated solution of PbCl2 increases its solubility
due to the formation of the complex ion [Pb(OH)3 ]− .
1. Write a balanced net ionic equation for this process.
2. The Ksp for PbCl2 = 1.7 x 10−5 ; Kf for [Pb(OH)3 ]− = 3.8 x 1014 . Calculate K for
this process.
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Chem 128, Exam III
April 25, 2002
Name
II. (42 points)
A. (12 points) Write your answers on the blanks provided. Use LT, GT, and EQ for less
than, greater than and equal to respectively, when appropriate.
1. At what temperature (in K) is ∆G◦ = ∆H◦ ? (1) .
2. What is the value of Q when ∆G◦ = ∆G? (2) .
3. So for steam is (3) (LT, GT, EQ) So for water.
4. The value for ∆G at equilibrium is (4) .
5. For a reaction, ∆H◦ < 0, ∆S◦ < 0, and ∆G◦ = 0 at 452 K. The
reaction is spontaneous at a temperature (5) (LT, GT, EQ)
452 K.
6. So for Ag(s) at 25◦ C is (5) (<, >, =) 0.
B. (6 points) Answer the questions below, using LT (for is less than ), GT (for is greater
than ), EQ (for is equal to ), or MI (for more information required ) in the blanks
provided.
The reaction given below takes place in a cylinder which feels warm to the touch after
the reaction is complete.
A2 (g) + B2 (g) → 2 AB (s)
1. At all temperatures, ∆S◦
0.
2. At all temperatures, ∆H◦
0.
3. At all temperatures, ∆G◦
0.
C. (6 points) Given the following molar entropies of formation (J/mol-K)
H2 O () = 69.9
NH3 (g) = 192.3
NO (g) = 210.7
Calculate ∆S◦ for the following reaction:
4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2 O ()
3
O2 (g) = 205.0
Chem 128, Exam III
April 25, 2002
Name
D. (18 points) For the reaction
2 CuCl (s) + 2 OH− (aq) → Cu2 O (s) + 2 Cl− (aq)
∆H◦ = – 54.3 kJ, and ∆S◦ – 125.1 J/K.
1. Calculate ∆G◦ at 200◦ C.
2. At 165◦ C, ∆G◦ for the reaction is 0.494 kJ. What is K for the reaction at 165◦ C?
3. Calculate Q and ln Q for the reaction if [OH− ] = 0.571 M and [Cl− ] = 0.876 M
ln Q =
Q=
4. At 165◦ C, and the conditions given in (3), what is ∆G for the reaction. Is it
spontaneous?
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Chem 128, Exam III
April 25, 2002
Name
III. (42 points)
A. (4 points) Write a balanced chemical equation for the cell represented by
2+
| (Pt)
Sn | Sn2+ || MnO−
4 | Mn
2+
half cell is in acidic medium.
The MnO−
4 | Mn
B. (16 points) Use the following data to answer the questions below:
Al3+ (aq) + 3 e− → Al (s)
Fe2+ (aq) + 2 e− → Fe (s)
Tl+ (aq) + e− → Tl (s)
Cu2+ (aq) + 2 e− → Cu (s)
Au3+ (aq) + 3 e− → Au (s)
E◦red
E◦red
E◦red
E◦red
E◦red
= – 1.68 V
= – 0.409 V
= – 0.336 V
= 0.339 V
= 1.498 V
Write your answers on the blanks provided with the best word
following list:
greater than
less than
equal to
false
unknown
electrolytic
oxidation
reduction
Nernst
You may use a term more than once.
or phrase from the
true
voltaic
Faraday
1. It is (1) that aluminum metal can reduce Au3+ .
2. Cu | Cu2+ || Au3+ | Au is a(n) (2) cell.
3. The E◦ox for gold metal is (3) –1 V.
4. E◦ox for Cu is (4) E◦red for Tl+ .
5. The ability of Tl to reduce Cu2+ is (5) the ability of Fe
to reduce Cu2+ .
6. The (6) constant has the value 96,480 J/mol-V.
Using the same data write the number required in the blank to make the statement
true.
7. E◦ for the cell Fe | Fe2+ || Au3+ | Au is (7) .
8. The coefficient for Tl in the balanced equation for the
cell Tl | Tl+ || Al3+ | Al is (8) .
5
Chem 128, Exam III
April 25, 2002
Name
C. (15 points) Consider the following reaction at 25◦ C:
+
2 NO−
3 (aq) + 3 Cu (s) + 8 H (aq)
→ 2 NO (g) + 3 Cu2+ (aq) + 4 H2 O
E◦ = 0.625 V
1. What is n?
2. What is ∆G◦ ?
3. What is K?
4. What is Q for the following conditions:
2+
[NO−
3 ] = 2.34 M, pH = 0, Cu = 2 g, [Cu ] = 0.021 M, NO = 1.00 atm
5. What is E under the conditions of (4)?
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Chem 128, Exam III
April 25, 2002
Name
D. (7 points) An electrolytic cell is made up of a 200.0 g strip of copper in 0.300 M
Cu(NO3 )2 and a 200.0 g strip of Cr in 0.300 M Cr(NO3 )3 . The overall reaction is:
3 Cu (s) + 2 Cr3+ (aq) → 3 Cu2+ (aq) + 2 Cr(s)
E◦ = – 1.083 V
An external battery provides 5 amperes for 4 hours and 10 minutes with 100%
efficiency.
1. How many coulombs are supplied by the battery?
2. What is the mass of the chromium strip after the battery is disconnected?
IV. (25 points) The last page of this exam packet is a schematic diagram for the qualitative
analysis of Groups I, II, and III ions. Tear that page out and write your answers to the
questions on the blanks below. The numbers below correspond to the answers to the
circled numbers on the diagram.
1.
2.
3.
4.
5.
6.
7.
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Chem 128, Exam III
April 25, 2002
Name
BONUS (15 points) All or nothing.
The bonus should be done only after you have completed the main part of this exam and
checked your work for errors. The time alloted for this exam does not include time for the
bonus. Trial and error solutions will not be accepted.
Ksp for silver chloride at 25◦ C is 1.782 x 10−10 . At 35◦ C, Ksp is 4.159 x 10−10 . What are
∆H◦ and ∆S◦ for the dissociation of AgCl?
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