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chem 302 exam 3 solution

chem 302 exam 3 solution - Version 125 Exam 3 Mccord(90540...

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Version 125 – Exam 3 – Mccord – (90540) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CHs302 001 10.0 points M 2+ is added to a solution of 0 . 0063 mol / L X - and 0 . 0016 mol / L Z 3 - . MX 2 has K sp = 5 . 1 × 10 - 12 , while M 3 Z 2 has K sp = 6 . 5 × 10 - 29 . What percentage of the Z - 3 is still in solution (not precipitated) when the MX 2 just starts to precipitate? 1. 3.68 2. 3.16 3. 3.88 4. 5.03 5. 3.01 6. 6.38 7. 6.22 8. 5.84 9. 8.36 10. 10.9 Correct answer: 10 . 9%. Explanation: K sp MX 2 = 5 . 1 × 10 - 12 [X - ] = 0 . 0063 mol / L K sp M 3 Z 2 = 6 . 5 × 10 - 29 [Z - 3 ] = 0 . 0016 mol / L The two relevant equilibria here are MX 2 (s) M 2+ + 2X - K sp = [M 2+ ] [X - ] 2 M 3 Z 2 (s) 3M +2 + 2Z 3 - K sp = [M 2+ ] 3 [Z 3 - ] 2 MX 2 would precipitate when K sp = [M 2+ ] [X - ] 2 5 . 1 × 10 - 12 = [M 2+ ] (0 . 0063 mol / L) 2 [M 2+ ] = 1 . 28496 × 10 - 7 mol / L This corresponds to a [Z 3 - ] of K sp = [M 2+ ] 3 [Z 3 - ] 2 6 . 5 × 10 - 29 = (1 . 28496 × 10 - 7 mol / L) 3 [Z 3 - ] 2 [Z 3 - ] = 0 . 000175034 mol / L The percentage of [Z 3 - ] still in solution is 0 . 000175034 mol / L 0 . 0016 mol / L (100 %) = 10 . 9% 002 10.0 points When ammonium chloride is added to NH 3 (aq), 1. the equilibrium concentration of NH 3 (aq) decreases. 2. the pH of the solution does not change. 3. the K b increases. 4. the pH of the solution increases. 5. the pH of the solution decreases. correct Explanation: 003 10.0 points Consider the fractional composition dia- gram for phosphoric acid where p K a1 = 2.12, p K a2 = 7.21, and p K a3 = 12.68. 0.2 0.4 0.6 0.8 1.0 2 4 6 8 10 12 14 pH Fractional Composition What is the structure of the dominant species at pH 3? 1. PO 3 - 4 2. HPO 2 - 4 3. H 3 PO 4 4. H 2 PO - 4 correct Explanation: To the left of 2 . 12, the red graph represent- ing H 3 PO 4 is dominant.
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Version 125 – Exam 3 – Mccord – (90540) 2 Between 2 . 12 and 7 . 21, the blue graph rep- resenting H 2 PO - 4 is dominant. Between 7 . 21 and 12 . 68, the orange graph representing HPO 2 - 4 is dominant. To the right of 12 . 68, the green graph rep- resenting PO 3 - 4 is dominant. 004 10.0 points What is the molar solubility of CuBr in 0.5 M NaBr? The K sp is 4 . 2 × 10 - 8 . 1. 4 . 20 × 10 - 8 2. 8 . 40 × 10 - 8 correct 3. 4 . 20 × 10 - 7 4. 3 . 48 × 10 - 3 5. 2 . 05 × 10 - 4 Explanation: 005 10.0 points You have available the following reagents as 0.10 M aqueous solutions: NaOH, HCl, HCN (p K a = 9.31), aniline (p K b = 9.13), HNO 2 (p K a = 3.25), and CH 3 NH 2 (p K b = 3.34). Which two reagents would you use to make the best buffer with a pH of 10.70? 1. HCl and CH 3 NH 2 correct 2. NaOH and HNO 2 3. NaOH and HCN 4. HCl and NaOH 5. HCl and aniline Explanation: The pKa for CH 3 NH + 3 is 10.66 which is the closest pKa to the desired buffer pH of 10.70.
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