solution_pdf - Version 288 Make Up Exam 2 VANDEN BOUT...

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Version 288 – Make Up Exam 2 – VANDEN BOUT – (53585) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points How long will it take For a current oF 3.00 amperes to plate 6 . 355 grams oF Cu out oF a solution oF Cu(NO 3 ) 2 ? (1 ±araday = 96,485 coulombs, Cu = 63 . 55 g/mol) 1. 53.6 min 2. None oF these 3. 2.68 hr 4. 12.9 sec 5. 26.8 min 6. 1.79 hr correct Explanation: I = 3.0 A m Cu = 6 . 355 g The reaction that is occuring is Cu 2+ + 2 e Cu(s) Number oF moles oF Cu(s) plated out 6 . 355 g 63 . 55 g / mol = 0 . 1 mol . Number oF moles oF e used 0 . 1 mol × 2 = 0 . 2 mol e . Number oF Coulombs = ±araday × moles oF e = (96485 C / mol) × (0 . 2 mol e ) = 19297 C . current = charge time 3 A = 19297 C time time = 19297 C 3 A = 6432 . 33 seconds 6432 seconds 6432 seconds = 6432 seconds 3600 seconds / hour = 1 . 78676 hrs 1 . 79 hrs 002 10.0 points A bu²er (oF pH 3.74) was prepared by mixing 1.00 mole oF Formic acid and 1.00 mole oF sodium Formate to Form an aqueous solution with a total volume oF 1.00 liter. To 250 mL oF this solution was added 10.0 mL oF 1.00 M NaOH. What is the pH oF this solution? 1. 4.29 2. 3.51 3. 3.78 correct 4. 4.08 5. 3.32 Explanation: [NaOH] = 1 M [H±] = 1 M ] = 1 M pH ini = 3.74 Initial condition (ini): n NaOH = 10 . 0 × 1 . 0 = 10 mmol n HF = 200 × 1 . 0 = 200 mmol n Na + = 250 × 1 . 0 = 250 mmol n F - = 250 × 1 . 0 = 250 mmol NaOH + H± Na + + ± + H 2 O ini 10 250 250 250 Δ - 10 - 10 10 10 fn 0 240 260 260 Na + is a spectator ion. H± / ± is a bu²er system. Since [H±] = [± ] in the original bu²er p K a = pH ini = 3 . 74. pH fn = p K a + log p b ± B [H±] P = 3 . 74 + log ± 260 240 ² = 3 . 77476 003 10.0 points A bu²er is Formed by mixing 100 mL oF 0.2 M
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Version 288 – Make Up Exam 2 – VANDEN BOUT – (53585) 2 HClO 2 and 200 mL of 0.7 M KClO 2 . What volume of 0.2 M KOH can be added before the buFer capacity is reached? 1. 300 mL 2. 100 mL correct 3. 10 mL 4. 700 mL 5. 150 mL Explanation: 004 10.0 points All components are present in 0.10 M concen- trations. I) HCN and NaCN II) NH 3 and NH 4 Cl III) HNO 3 and NH 4 NO 3 IV) HClO 3 and NaClO 3 Which will give buFer solutions? 1. I and II only correct 2. I, III and IV only 3. I and III only 4. II, III and IV only 5. III and IV only Explanation: BuFers are formed in one of two ways, by combining a weak acid and its conjugate base or by combining a weak base and its conjugate acid. HNO 3 and HClO 3 are both strong acids and cannot be used to make eFective buFer solutions. HCN is a weak acid and NaCN is the salt of
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This homework help was uploaded on 04/12/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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solution_pdf - Version 288 Make Up Exam 2 VANDEN BOUT...

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