EXAM #2
Version 1
CHEM 3369 4:00–5:30 pm This exam contains 35 questions. Mark the choice on the answer sheet corresponding to the best answer for each of the following questions. You can neglect the activity coefficients in all the questions. 1.
Which solution contains the lowest concentration of hydronium ions? A. B. C. D. E.
2.
What is the hydronium ion concentration of a 0.00086 M nitric acid? A. B. C. D. E.
3.
3.9x10-4 M 6.6x10– 7 M 3.9x10– 8 M 5.7x10– 4 M 4.3x10–12 M
Which of the following acids is the weakest acid? Each is listed with its Ka. A. B. C. D. E.
5.
1.9x10–12 M 1.16x10–11 M 0.0104 M 8.6x10– 4 M 4.7x10– 1 M
What is the hydroxide ion concentration of a solution with pH = 7.82? A. B. C. D. E.
4.
0.3 M NH2 NH2 0.3 M KOH 0.3 M Ba(OH)2 0.3 M NH3 0.3 M (C2 H5 )3 N
HF, 3.5x10– 4 CH3 COOH, 1.8x10– 5 HBrO, 2.0x10– 9 HClO, 3.0x10– 5 H2 SO3 , 1.1x10– 2
Two of the equilibria shown are autoionizations. Which one is not? A. H2 O + H2 O ´ H3 O+ + OH– B. H2 O + H2 O ´ H2 O2 + H2 C. NH3 + NH3 ´ NH4 + + NH2 –
6.
A 0.100 M solution of the weak acid HA has a pH of 2.36. Calculate pKa for HA? A. B. C. D. E.
7.
2.36 10.30 3.70 7.00 1.00
What is the pH of the solution that results when 10.0 mL of 0.0835 M HCl and 10.0 mL of 0.0850 M NaOH are mixed? A. B. C. D. E.
10.88 7.00 1.07 12.93 3.12
Questions 8–10 concern a solution of 3.0x10– 5 Ba(OH)2 , which is completely dissociated to Ba2+ and OH– in water. 8.
A charge balance for this system is: A. B. C. D. E.
9.
2[Ba2+] + [H+] = [OH– ] [Ba2+] + [H+] = [OH– ] [Ba2+] + [H+] = 2[OH– ] 2[Ba2+] = [OH– ] [Ba2+] = 2[OH– ]
A mass balance for this system is: A. B. C. D. E.
[Ba2+] + [H+] = [OH– ] [Ba2+] = 2[OH– ] [Ba2+] = 1.5x10– 5 [OH– ] = 6.0x10– 5 [Ba2+] = 3.0x10– 5
10. Calculate the pH of this solution. A. B. C. D. E.
11.73 9.78 4.22 4.52 9.48
11. The equilibrium HBrO(aq) + H2 O(l) ´ H3 O+(aq) + BrO– (aq) forms some products but strongly favors reactants. What is the strongest acid in the equilibrium? A. B. C. D.
H2 O HBrO BrO– H3 O+
12. What is the strongest Bronsted base that can exist in water? A. B. C. D. E.
NH2 – OH– NH2 NH2 NH3 CH3 –
13. The autoionization constant, Kw, for pure water at 40o C is 2.9x10–14. What is the pH of pure water at 40o C? A. B. C. D. E.
7.00 6.46 6.77 6.54 7.23
14. What is the pKb of a base with Kb = 8.2x10– 8? A. B. C. D. E.
8.91 –7.09 6.31 7.09 –8.91
15. What is the pH of a 0.0010 M H2 SO4 (aq) solution? Ka2 = 0.012. A. B. C. D. E.
3.00 2.73 1.54 1.74 2.22
16. Ethylamine, C2 H5 NH2 , is a base? What is the equilibrium that occurs when it is placed in water? A. B. C. D. E.
C 2 H5 NH2 (aq) + H2 O(l) ´ C 2 H5 NH2 (aq) ´ NH2 – (aq) C 2 H5 NH2 (aq) + H2 O(l) ´ C 2 H5 NH2 (aq) + H2 O(l) ´ none of the above is correct
H3 O+(aq) + C2 H5 NH– (aq) + C2 H5 +(aq) OH– (aq) + C2 H5 NH3 +(aq) OH– (aq) + C2 H5 NH+(aq) H2 (g)
17. What is the pH of a solution 0f 0.050 M calcium acetate, Ca(CH3 COO)2 ? pKa(CH3 COOH) = 4.75. A. B. C. D. E.
10.32 5.28 8.72 3.74 7.66
Questions 18–23 concern a titration in which 20.00 mL of 0.145 M NaOH are titrated with 0.100 M HCl. 18. Calculate the equivalence volume. A. B. C. D. E.
20.00 40.00 19.00 15.00 29.00
mL mL mL mL mL
19. What is the pH before any HCl has been added? A. B. C. D. E.
13.04 7.00 13.16 1.06 0.84
20. What is the pH after addition of 15.00 mL of HCl? A. B. C. D. E.
1.20 12.60 7.00 1.44 12.85
21. What is the pH at the equivalence point? A. B. C. D. E.
1.24 5.83 8.26 7.00 12.81
22. What is the pH after the addition of a total of 40.00 mL HCl? A. B. C. D. E.
1.74 12.46 7.00 1.06 12.81
23. Which indicator of the following five would be the best for this titration? Each indicator is shown with the transition range (pH). A. B. C. D. E.
thymol blue (8.0–9.6) methyl orange (3.1–4.4) congo red (3.0–5.0) phenol red (6.4-8.0) cresol red (7.2–8.8)
24. What is the percentage hypobromite ion (BrO– ) protonated in a 0.16 M solution of sodium hypobromite? K a(HBrO) = 3.0x10– 5. A. B. C D. E.
0.019% 3.0% 4.6x10– 3% 2.1x10– 7% 2.7%
25. A solution contains 63 different conjugate acid-base pairs. Among them is acrylic acid (H2 C=CHCOOH, pKa = 4.25) and acrylate ion, with the ratio [acrylate]/[acrylic acid] = 0.75. What is the pH of the solution? A. B. C. D. E.
5.02 4.53 4.13 4.37 4.25
26. What reaction occurs in the HF–NaF buffer when a strong base is added? A. B. C. D. E.
HF(aq) + H2 O(l) ´ H3 O+(aq) + F– (aq) OH– (aq) + NaF(aq) ´ NaOH(aq) + F– (aq) OH– (aq) + HF(aq) ´ H2 O(l) + F– (aq) OH– (aq) + F-(aq) ´ O2 –(aq) + HF(aq) NaF(aq) + H2 O(l) ´ HF(aq) + NaOH(aq)
27. In titration of a weak base with a strong acid, the solution at the stoichiometric point is: A. B. C. D. E.
always basic always acidic always neutral always a buffer sometimes basic, sometimes acidic, sometimes neutral, and sametimes a buffer
28. Would the indicator bromocresol green (pKHIn = 4.7; acid color, yellow; base color, blue) be useful in the titration of a weak acid with a strong base? A. B. C. D.
yes, if pKa = 4.7±1.0 no, if pK a = 4.7±1.0 yes, always no, never
29. Which of the five one should apply to promote particle growth in gravimetric analysis? A. B. C. D. E.
increase the solubility by lowering the temperature raise the temperature to increase relative supersaturation raise the temperature to decrease relative supersaturation keep the volume small so that concentrations of analyte and precipitant are high always use a buffer
30. At what pH would you expect the aminoacid proline (P) to be fully protonated? pK1 = 1.952, pK2 = 10.640. A. B. C. D. E.
1.952 1.750 2.500 10.640 7.000
31. Given pKa values of citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid), pK1 = 3.128, pK2 = 4.761, and pK3 = 6.396, calculate the first base association constant for sodium citrate. A. B. C. D. E.
7.45 1.73 2.49 4.02 1.34
x x x x x
10– 4 10– 5 10– 8 10– 7 10–11
32. The aminoacid tyrosine (Y) has three acidic protons in fully protonated form: H3 Y+ (pK1 = 2.17) Æ H2 Y (pK2 = 9.19) Æ HY– (pK3 = 10.47) Æ Y2 –. Calculate the pH of 0.0150 M tyrosine. A. B. C. D. E.
2.17 5.68 6.34 1.82 10.47
Questions 33–35 concern a diprotic acid H2 A with pK1 = 4.00 and pK2 = 8.00. 33. Which is the principal species at pH 6.00? A. H2 A B. HA– C. A2 – 34. At what pH is [H2 A] = [HA– ] A. B. C. D. E.
7.00 4.00 8.00 6.00 2.00
35. At what pH is [HA– ] = [A2 –] A. B. C. D. E.
7.00 4.00 8.00 6.00 2.00
