Worksheet 10 Key

Worksheet 10 Key - CH 302 Spring 2007 Practice Exam 2 1...

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CH 302 Spring 2007 Practice Exam 2 1. Which of the following is not a strong acid? a. HCl b. H 2 SO 4 c. HClO 4 d. H 2 CO 3 e. HClO 3 2. What is the pH of a buffer made up of 0.5 M pyridine (C 5 H 5 N) and 0.75 M C 5 H 5 NH + ? The pK b of pyridine is 8.70. a. 8.9 b. 6.4 c. 5.1 d. 7.6 Answer: This shouldn’t require any real calculation. 3. Which of the following systems produces a buffer? I. 0.5 M CH 3 COOH and 0.5 M NaCH 3 COO II. 0.5 M CH 3 COOH, 1.0 M NaCH 3 COO, and 0.25 M Ba(OH) 2 III. 0.75 M CH 3 COOH and 0.5 M NaOH IV. 1.0 M Na + and 1.0 M NaOH V. 0.5 M CH 3 COOH, 1.0 M NaCH 3 COO, and 0.5 M HCl a. I, II, III, IV, and IV b. None of them c. II, III, and V only d. I only e. I, II, and IV only f. I and II only Answer: Remember to neutralize. 4. Rank the following compounds in terms of increasing basicity. Compound pK b C 6 H 5 NH 2 9.38 NH 3 4.74 HONH 2 8.18 (CH 3 )N 4.13 CH 3 NH 2 3.30 a. C 6 H 5 NH 2 < HONH 2 < NH 3 < (CH 3 )N < CH 3 NH 2 b. CH 3 NH 2 < (CH 3 )N < NH 3 < HONH 2 < C 6 H 5 NH 2 c. C 6 H 5 NH 2 < HONH 2 < (CH 3 )N < NH 3 < CH 3 NH 2 d. CH 3 NH 2 < NH 3 < (CH 3 )N < HONH 2 < C 6 H 5 NH 2 Answer: Remember that larger K b => smaller pK b 5. What is the buffer capacity of a buffer made up of 1.2 M CH 3 NH 2 and 1.4 M CH 3 NH 3 + ? For CH 3 NH 2 , pK b = 3.3. a. 1.4 M for H + , 1.2 M for OH - b. 1.2 M for H + , 1.4 M for OH - c. 0.2 M for H + , 0.2 M for OH - d. 2.6 M for H + , 2.6 M for OH - Answer: The CH 3 NH 2 can react with 1.2 M of H + ; the CH 3 NH 3 + can react with 1.4 M OH - . 6. 100 mL of 1.5 M HCN (pK a = 9.40) and 75 mL of 2.5 M CN - are mixed together, and 2 g of NaOH are added. What is the final pH?
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a. 4.60 b. 9.13 c. 9.40 d. 9.77 e. 13.25 Answer: This shouldn’t require any real calculation. 7. Identify the equivalence points on this titration curve. a. B, D, F, and H b. B, D, and F c. C, E, and J d. C and E only e. H only 8. On the same titration curve referred to in question 7, at which points might you use the equation for an amphiprotic acid? Assume the pH does not “jump” again as more titrant is added. a. B, D, F, and H b. B, D, and F c. C, E, and J d. C and E only e. H only Answer: At point J, we have A 3- , which is not amphitprotic; it’s just a weak base. 9.
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This note was uploaded on 05/02/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.

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Worksheet 10 Key - CH 302 Spring 2007 Practice Exam 2 1...

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