Chem 112, Fall 05
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Exam 3A
Before you begin, make sure that your exam has all 10 pages.
There are 32 required problems (3 points each, unless noted otherwise) and two extra credit
problems (3 points each).
Stay focused on your exam.
YOU MUST:
Put your name and student ID on the bubble sheet correctly.
Put all your answers on the bubble sheet; nothing on this exam will be used for grading.
Sign the statement on the last page of the exam.
Turn in both the exam and bubble sheet when you are done. Good Luck!
Some values of Ksp
Carbonates
Hydroxides
BaCO3
8.1 x 10-9
NiCO3
6.6 x 10-9
Ni(OH)2 2.8 x 10-16
Fe(OH)2 7.9 x 10-15
Fe(OH)3 6.3 x 10-38
page 1
Sulfates
BaS04
1.1 x 10-10
Sulfides
CaS
8 x 10-6
NiS
3.0 x 10-21
Chem 112, Fall 05
Name:________________________________
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Exam 3A
1. According to the Brønsted-Lowry definition, a base
A.
B.
C.
D.
E.
increases the H3O+ concentration in an aqueous solution.
increases the OH- concentration in an aqueous solution.
is a proton acceptor.
is a proton donor.
is an electron-pair donor.
2. Which of the following is never a Brønsted-Lowry acid in an aqueous solution?
A.
B.
C.
D.
E.
hydrogen chloride, HCl(g)
dihydrogen sulfide, H2S(g)
ammonium chloride, NH4Cl(s)
hydrogen fluoride, HF(g)
magnesium oxide, MgO(s)
3. Which salt forms a 0.10 M aqueous solution with the highest pH?
A. NaNO3
B. NH4Cl
C. FeCl3
D. Ca(ClO4)2
E. NaF
4. Consider the following series of acids:
CH3CO2H, ClCH2CO2H, Cl2CHCO2H, Cl3CCO2H
Which statement explains the trend in the pKAs (left to right).
A.
B.
C.
D.
E.
pKAs increase due to inductive effects generated by the substitution of H with Cl.
pKAs decrease due to inductive effects generated by the substitution of H with Cl.
pKAs increase due to the larger number of resonance structures possible in the anion.
pKAs decrease due to the larger number of resonance structures possible in the anion.
Both B and D.
page 2
Chem 112, Fall 05
Name:________________________________
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Exam 3A
5. At 15 °C, the water ionization constant, Kw, is 4.5 × 10-15. What is the H3O+ concentration in
neutral water at this temperature?
A.
B.
C.
D.
E.
2.0 × 10-29 M
4.5 × 10-15 M
6.7 × 10-8 M
1.5 × 10-7 M
2.2 M
6. If you mix equal molar quantities of HF (Ka = 7.2 × 10-4) and KCN (Kb = 2.5 × 10-5), the
resulting solution will be
A.
B.
C.
D.
E.
acidic because Ka of HF is greater than Kb of CN-.
basic because Ka of HF is greater than Ka of HCN.
basic because Ka of HF is greater than Kb of CN-.
basic because Kb of F- is less than Kb of CN-.
neutral because the weak acid neutralizes the weak base.
7. Determine the equilibrium constant for the reaction below. (Ka (HF) = 7.2 × 10-4,
Ka (NH4+) = 5.6 × 10-10)
HF(aq) + NH3(aq) ' NH4+(aq) + F-(aq)
A. 4.0 × 10-13
B. 1.3 × 10-8
C. 7.8 × 10-7
D. 1.3 × 106
E. 2.5 × 1012
8. Which of the following chemical equations corresponds to Ka3 for phosphoric acid?
A.
B.
C.
D.
E.
HPO42-(aq) + H2O(l) ' PO43-(aq) + H3O+(aq)
PO43-(aq) + H2O(l) ' HPO42-(aq) + OH-(aq)
H3PO4(aq) + H2O(l) ' H2PO4-(aq) + H3O+(aq)
H3PO4(aq) + 3 H2O(l) ' PO43-(aq) + 3 H3O+(aq)
H2PO4-(aq) + H2O(l) ' HPO42-(aq) + H3O+(aq)
page 3
Chem 112, Fall 05
Name:________________________________
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Exam 3A
9. Boric acid (H3BO3) has a pKa value of 9.14. What is the Kb of sodium borate (NaH2BO3) at
25 oC?
A. 7.2 × 10-10
B. 1.4 × 10-5
C. 1.1 ×10-4
D. -4.86
E. 4.86
10. In the following acid-base reaction equilibrium
C9H7NH+ + C6H5COO- ' C9H7N + C6H5COOH
+
A. C9H7NH is the conjugate acid of C9H7N.
B. C6H5COO- is the conjugate acid of C6H5COOH.
C. C6H5COOH and C6H5COO- represent a conjugate acid-base pair
D. Both A and B
E. Both A and C
11. Suppose that K < 1 for the equilibrium in problem 10.
+
A. C6H5COO- is the weaker base and C9H7NH is the stronger acid
+
B. C6H5COO- is the weaker base and C9H7NH is the weaker acid
C. C9H7N is the weaker base and C6H5COOH is the weaker acid
D. C9H7N is the stronger base and C6H5COOH is the stronger acid
E. Both B and D
12. (4 pts) For the following three reactions, K > 1
HF (aq) + C6H5COO- (aq) '
HC9H7O4 (aq) + C6H5COO- (aq) '
HF (aq) + C9H7O4- (aq)
F- (aq) + C6H5COOH (aq)
C6H5COOH (aq) + C9H7O4- (aq)
' F- (aq) + HC9H7O4 (aq)
Among the three acids and three conjugate bases, which are the strongest acid and base?
A.
B.
C.
D.
E.
HC9H7O4 and C6H5COOHF and C6H5COOHF and C9H7O4C6H5COOH and FC9H7CO4- and F-
page 4
Chem 112, Fall 05
Name:________________________________
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Exam 3A
13. Two solutions of the same weak acid (pKA = 6) were made to have different concentrations
(10-4 M and 0.1 M). The 10-4 M solution was found to have a
A.
B.
C.
D.
smaller pH than the 0.1 M solution
larger pH than the 0.1 M solution
the same pH as the 0.1 M solution
Not enough information is provided to determine if a difference in pH exists.
14. Two buffer solutions were made with equal molar mixtures of a weak acid its conjugate
base, and differed only by the total concentration of acid plus base. The buffer solution with
the larger total acid plus base concentration had
A.
B.
C.
D.
a pH that was larger than the buffer with the smaller total concentration.
a pH that was smaller than the buffer with the smaller total concentration.
a pH that was the same as the buffer with the smaller total concentration.
Not enough information is provided to make this decision.
15. Each of the following mixtures can produce a buffer solution EXCEPT
A.
B.
C.
D.
E.
HClO4 and NaClO4
HF and NaF
NaHCO3 and Na2CO3
Na2HPO4 and Na3PO4
NH4Cl and NH3
16. Which of the following mathematical expressions is the Henderson-Hasselbalch equation?
⎛ [ conjugate base] ⎞
A. pK a = pH + log ⎜⎜
⎟⎟
[acid ]
⎝
⎠
⎛ ⎡OH - ⎤ ⎞
⎦ ⎟
B. pH = pK a + log ⎜ ⎣
⎜ ⎡ H 3O + ⎤ ⎟
⎦⎠
⎝⎣
⎛
⎞
[acid ]
C. pH = pK a + log ⎜
⎜ [ conjugate base] ⎟⎟
⎝
⎠
⎛
⎞
[acid ]
D. pK a = pH - log ⎜
⎜ [ conjugate base] ⎟⎟
⎝
⎠
⎛ [ conjugate base] ⎞
E. pH = pK a + log ⎜
⎟⎟
⎜
[acid ]
⎝
⎠
page 5
Chem 112, Fall 05
Name:________________________________
(Print Clearly)
Exam 3A
17. What is the pH of a solution that results from adding 25 mL of 0.15 M HCl to 25 mL of
0.52 M NH3? (Kb of NH3 = 1.8 × 10-5)
A. 2.74
B. 4.35
C. 9.65
D. 11.26
E. 11.41
18. Which of the following acid-base pairs is most suited to make a buffer with a pH = 7.68?
A.
B.
C.
D.
E.
HCO2H/NaHCO2
H2CO3/NaHCO3
HOCl/NaOCl
H3BO3/NaH2BO3
NH4Cl/NH3
pKA
pKB
3.74
6.38
7.46
9.14
9.25
10.26
7.62
6.54
4.86
4.75
19. 500 mL of a buffer solution formed with NH4Cl and NH3 has a pH = 9.64 and the
[NH3] = 0.5 M. What is the number of grams of NH4Cl required to make this solution?
(MWNH3 = 17.0 g/mol, MWNH4Cl = 53.5 g/mol) (Kb of NH3 = 1.8 × 10-5)
A.
B.
C.
D.
E.
10.90 g
1.73 g
3.46 g
5.57 g
26.7 g
20. If the ratio of acid to base in a buffer increases by a factor of 10, the pH of the buffer
A. decreases by 1
D. increases by 1
B. decreases by 10
E. remains unchanged
C. increases by 10
21. What is the pH of the buffer that results when 11 g of NaCH3CO2 is mixed with 85 mL of
1.0 M CH3CO2H and diluted with water to 1.0 L? (Ka of CH3CO2H = 1.8 × 10-5; MW of
CH3CO2Na = 82.04 g/mol)
A. 2.91
B. 3.86
C. 4.55
page 6
D. 4.74
E. 4.94
Chem 112, Fall 05
Name:________________________________
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Exam 3A
22. Which one of the following conditions is always met at the equivalence point of the titration
of a monoprotic weak base with a strong acid?
A.
B.
C.
D.
E.
The pH of the solution is equal to 7.00.
The volume of acid added from the buret equals the volume of base titrated.
The molarity of the acid equals the initial molarity of the weak base.
The percent ionization of the acid equals the percent ionization of the base.
The moles of acid added from the buret equals the initial moles of weak base.
23. Which one of the following conditions is always true for a titration of a weak acid with a
strong base?
A.
B.
C.
D.
E.
A colored indicator with a pKa less than 7 should be used.
If a colored indicator is used, it must change color rapidly in the weak acid’s buffer region.
Equal volumes of weak acid and strong base are required to reach the equivalence point.
The equivalence point occurs at a pH equal to 7.
The equivalence point occurs at a pH greater than 7.
24. A 25.0 mL sample of 0.0200 M NH3(aq) is titrated with 0.0100 M HCl(aq). What is the pH
at the equivalence point? (Kb of NH3 = 1.8 × 10-5)
A. 3.46
B. 5.48
C. 5.72
D. 8.25
E. 10.54
25. Which of the following equations is the solubility product for magnesium iodate, Mg(IO3)2?
A. K sp = [Mg 2+ ][I- ]2 [O 2- ]6
B. K sp = [Mg 2+ ][I- ]2 [3O 2- ]2
C. K sp = [Mg 2+ ][IO3- ]
D. K sp = [Mg 2+ ]2 [IO3- ]
E. K sp = [Mg 2+ ][IO3- ]2
page 7
Chem 112, Fall 05
Name:________________________________
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Exam 3A
26. The solubility of SrSO4 in water is 0.107 g in 1.0 L at 25 °C. What is the value of Ksp for
SrSO4?
A. 3.4 × 10-7
B. 5.8 × 10-4
C. 1.2 × 10-3
D. 1.1 × 10-2
E. 2.1 × 10-1
27. What is the molar solubility of Fe(OH)3(s) in a solution that is buffered at pH 2.75 at 25 °C?
The Ksp of Fe(OH)3 is 6.3 × 10-38 at 25 °C.
A.
B.
C.
D.
E.
1.1 × 10-29 mol/L
1.1 × 10-26 mol/L
2.0 × 10-15 mol/L
2.2 × 10-10 mol/L
3.5 × 10-4 mol/L
28. The following anions can be separated by precipitation as silver salts: Cl-, Br-, I-, CrO42-. If
Ag+ is added to a solution containing the four anions, each at a concentration of 0.10 M, in
what order will they precipitate?
Compound
AgCl
Ag2CrO4
AgBr
AgI
A.
B.
C.
D.
E.
Ksp
1.8 × 10-10
1.1 × 10-12
5.4 × 10-13
1.5 x 10-16
AgCl → Ag2CrO4 → AgBr → AgI
AgI → AgBr → Ag2CrO4 → AgCl
Ag2CrO4 → AgCl → AgBr → AgI
Ag2CrO4 → AgI → AgBr → AgCl
AgI → AgBr → AgCl → Ag2CrO4
page 8
Chem 112, Fall 05
Name:________________________________
(Print Clearly)
Exam 3A
29. (4 pts) Barium sulfite (s) and barium fluoride (s) are in equilibrium with a solution
containing 8.70 x 10-3 M ammonium fluoride (completely soluble). Calculate the
concentration of sulfite ion present in this solution. Ksp (BaSO3) = 6.0 x 10-7, Ksp (BaF2) =
1.7 x 10-6.
A.
B.
C.
D.
E.
0.35 M
0.023 M
2.7 x 10-5 M
3.1 x 10-3 M
6.0 x 10-7 M
30. A solution contains 0.10 M potassium sulfide and 0.10 M sodium carbonate. Solid nickel
acetate is added slowly to this mixture. What substance precipitates first?
A.
B.
C.
D.
E.
K2CO3
NiS
CH3CO2K
NiCO3
No precipitate will form.
31. The ionic solids AgNO3 and KI are dissolved in water and mixed together and in various
proportions to form solutions with the Ag+ and I- concentrations listed below:
1
2
3
[Ag+]
(M)
10-5
10-8
10-16
[I-]
(M)
10-10
10-8
2
Which of these solutions will form a precipitate? (Ksp = 1.5 x 10-16)
A.
B.
C.
D.
E.
1
1 and 3
1 and 2
all three
None of the solutions form precipitate.
page 9
Chem 112, Fall 05
Name:________________________________
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Exam 3A
32. (4pts) A solution contains 6.8 x 10-4 M KOH. Solid iron(III) nitrate is added slowly to this
mixture. What is the concentration of iron(III) ion when precipitation first begins ?
A.
B.
C.
D.
E.
6.8 x 10-4 M
1.2 x 10-11 M
1.4 x 10-31 M
2.0 x 10-28 M
1.7 x 10-8 M
EXTRA CREDIT
33. (3 pts) At 25 °C, only 1.04 x 10-3 g Cu(NO3)2 will dissolve per liter of a solution that is
buffered at pH 6.80. What is the value of Ksp for Cu(OH)2? The molar mass of Cu(NO3)2 is
187.6 g/mol.
A. 2.2 × 10-20
B. 1.4 × 10-19
C. 4.1 × 10-18
D. 3.5 × 10-13
E. 8.8 × 10-13
34. (3 pts) What is the concentration of PO43- in a solution created by mixing equal volumes of
0.1 M NaH2PO4 and 0.2 M Na2HPO4? (The acid dissociation constants for phosphoric acid
are Ka1 = 7.5 × 10-3, Ka2 = 6.2 × 10-8, and Ka3 = 3.6 × 10-13)
A. 6.2 × 10-8 M B. 0.2 M
C. 2.3 × 10-6
D. 5.8 × 10-5
E. zero
Please sign the following statement at the completion of the exam.
I, ________________________________________________(print name), have completed this
exam honestly, and pledge not to discuss it with anyone who has not taken the exam until it
is graded and returned.
_________________________________________________
(signature)
page 10
Chem 112, Fall 05
Name:________________________________
(Print Clearly)
Exam 3A
page 11