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Chemistry 116: Exam 3
March 21, 2017
1.
Given that KB of CH3NH2 is 4.2×10–4 at 25oC, what is the equilibrium constant for the reaction
CH3NH3+(aq) + H2O(l) ←→ CH3NH2(aq) + H3O+(aq)
a.
b.
c.
d.
2.4×10–11
4.2×10–10
4.2×10–4
1.0×10−14
2. Buffer solutions with the component concentrations shown below were prepared. Which of them
should have the lowest pH?
a. [H2PO4–] = 0.80 M, [HPO42–] = 0.80 M
b. [H2PO4–] = 0.40 M, [HPO42–] = 0.80 M
c. [H2PO4–] = 0.75 M, [HPO42–] = 1.0 M
d. [H2PO4–] = 1.0 M,
[HPO42–] = 1.0 M
–
e. [H2PO4 ] = 1.0 M,
[HPO42–] = 0.50 M
3. What is the pH of a 0.50 M solution of hypobromous acid (HOBr, KA= 2.5×10–9)
a. 3.54
b. 0.30
c. 4.45
d. 4.30
e. 4.90
4. A 0.14 M aqueous solution of an acid HA has a pH of 3.40. What is the value of KA for this acid?
a. 4.0×10−4
b. 1.1×10−6
c. 8.4×10−6
d. 1.6×10−7
e. 8.8×10−7
5. Which of the following pKA values corresponds to the weakest acid?
a. pKA = 2.43
b. pKA = 4.55
c. pKA = 8.23
d. pKA = 11.89
6. Calculate the pH of 0.036 M sodium cyanide, NaCN, solution (for HCN, KA = 6.2×10−10).
a. 10.65
b. 10.88
c. 8.67
d. 9.70
e. 9.40
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
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Chemistry 116: Exam 3
March 21, 2017
7. A 0.125 M solution of the weak acid HA is 9.2% dissociated. What is the pH of the solution?
a. 1.13
b. 0.94
c. 0.90
d. 1.94
e. none of the above
8. A buffer solution with a pH of 8.00 is needed in an experiment. Which of the following buffer
systems would work best?
a. H2PO4‒ (Ka = 6.2×10‒8) / HPO42‒
b. NH3 (Kb = 1.8×10‒5) / NH4+
c. N2H4 (Kb = 9.6×10‒7) / N2H5+
d. H2CO3 (Ka = 4.3×10‒7) / HCO3‒
e. HNO2 (Ka = 7.1×10‒4) / NO2‒
9. What is the pH of 0.20 M sulfurous acid (H2SO3) solution. KA1= 1.2×10−2 and KA2= 6.6×10−8
a. 0.70
b. 2.34
c. 1.31
d. 1.36
e. 0.96
10. Which one of the following substances will give an aqueous solution of pH > 7?
a. NH4Br
b. NO2
c. NaClO2
d. CH3OH
e. KI
11. Consider a solution prepared by dissolving 0.35 moles of CH3NH3Cl in 1.00 L of 1.1 M CH3NH2.
If 0.004 moles of NaOH is added to this buffer solution, the added NaOH reacts with the
_________ present in the solution, and the pH of the solution will _________ slightly.
a. H3O+
increase
b. CH3NH2
increase
c. CH3NH3+
increase
d. CH3NH2
decrease
+
e. CH3NH3
decrease
12. Select the correct relationship among the concentrations of species present in a 1.0 M aqueous
solution of the weak base represented by B (its conjugate base is represented by BH+).
a. [BH+] ~ [OH−] > [B] > [H3O+]
b. [B] > [BH+] > [OH−] > [H3O+]
c. [BH+] > [H3O+] > [B] > [OH−]
d. [B] > [BH+] ~ [OH−] > [H3O+]
e. [B] > [BH+] ~ [H3O+] > [OH−]
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
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Chemistry 116: Exam 3
March 21, 2017
13. An acetic acid buffer containing 0.60 M CH3COOH (KA=1.8×10−5) and 0.60 M CH3COONa was
prepared. What will the pH be after 0.007 mol of HCl has been added to 100.0 mL of the buffer?
a. 4.64
b. 4.81
c. 4.78
d. 4.85
e. 4.74
14. Which of the following acids has the strongest conjugate base?
name
pKA
a. Carbonic acid
6.37
b. Sulfurous acid
6.91
c. Arsenic acid
7.10
d. Phosphoric acid
7.21
e. Water
7.00
15. A solution is prepared by adding 0.10 mol of sodium fluoride, NaF, to 1.00 L of water. Which
statement about the solution is correct?
a. The concentrations of fluoride ions and sodium ions will be identical
b. The solution is basic
c. The solution is neutral
d. The solution is acidic
e. Both a and b
16. Which statement about buffers is true?
a. Buffers consist of a strong acid and its conjugate base.
b. A buffer does not change pH upon addition of a strong acid or strong base.
c. Buffers resist change in pH upon addition of small amounts of strong acid or strong base.
d. Buffers have a pH = 7.
e. Both a and c
17. What is the conjugate base of HPO42−?
a. H3O+
b. H2PO4−
c. OH−
d. H3PO4
e. PO43−
18. The ionization constant of water (KW) at 50oC is 5.5×10–14. What is the pH of pure water at this
temperature? Hint; What is the definition of neutral water?
a. 5.50
b. 6.50
c. 6.63
d. 7.00
e. 7.50
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
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Chemistry 116: Exam 3
March 21, 2017
19. Calculate the pH of a 0.0048 M solution of Ba(OH)2
a. 12.03
b. 2.32
c. 11.68
d. 11.98
e. 2.02
20. Which of the following mixture will produce a buffer solution if one liter of each solution is mixed
together?
a. 0.80 M HNO3 and 1.60 M NaNO3
b. 1.2 M NH3 and 0.70 M HCl
c. 0.8 M HC2H3O2 and 1.4 M NaOH
d. 0.80 M HF and 1.20 M NaOH
21. Each of the following pairs contains one strong acid and one weak acid except
a. HBr and H3PO4
b. HNO3 and H2CO3
c. HF and HNO2
d. H2SO4 and H2S
e. HC2H3O2 and HCl
Key (versions a and d)
1. A
4. B
2. E
5. D
3. C
6. B
7. D
8. C
9. D
10. C
11. C
12. D
13. A
14. D
15. B
16. C
17. E
18. C
19. D
20. B
21. C
Key (versions b and e)
1. C
4. D
2. E
5. B
3. C
6. C
7. C
8. D
9. C
10. C
11. A
12. E
13. C
14. B
15. D
16. B
17. D
18. D
19. A
20. D
21. B
Key (versions c and f)
1. B
4. C
2. D
5. D
3. B
6. C
7. E
8. C
9. D
10. D
11. A
12. D
13. B
14. C
15. E
16. C
17. D
18. B
19. C
20. C
21. A
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
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