Makeup Exam 2, Spring 2011

# Makeup Exam 2, Spring 2011 - Version 106 Make up 2...

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Version 106 – Make up 2 – laude – (51635) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 6.0 points Na±, NaCl, and HBr are dissolved in water. How many equations are needed to describe this system? 1. 7 correct 2. 5 3. 6 4. 4 5. 8 Explanation: The species Na + , H±, ± - , Cl - , Br - , H + , and OH - will be present in the water. 002 6.0 points A solution initially contains 0 . 04 M Sb 3+ and 0 . 03 M Pb 2+ . Some amount oF hydrogen sul- fde (H 2 S) is added to the solution, and the S 2 - concentration is controlled by adjusting the pH. Sb 2 S 3 and PbS will start to precip- itate at which oF Following values oF [S 2 - ], respectively? Sb 2 S 3 : K sp = 1 . 6 × 10 - 93 PbS: K sp = 3 × 10 - 28 1. 1 × 10 - 90 M; 1 × 10 - 26 M 2. 4 × 10 - 92 M; 1 × 10 - 26 M 3. 1 × 10 - 26 M; 1 × 10 - 30 M 4. 1 × 10 - 30 M; 1 × 10 - 26 M correct 5. 1 . 6 × 10 - 93 M; 3 × 10 - 28 M Explanation: ±or PbS, K sp = [Pb 2+ ] [S 2 - ] 3 × 10 - 28 = (0 . 03 M) [S 2 - ] [S 2 - ] = 1 × 10 - 26 M ±or Sb 2 S 3 , K sp = [Sb 3+ ] 2 [S 2 - ] 3 1 . 6 × 10 - 93 = (0 . 04 M) 2 [S 2 - ] 3 [S 2 - ] = 1 × 10 - 30 M 003 6.0 points ±or a weak diprotic acid system (H 2 A) which oF the Following statements are true? I) H 2 A and A 2 - Form a bu²er. II) K a1 > K a2 III) [A 2 - ] > [HA - ] in acidic solution IV) HA - is amphiprotic. 1. II and IV only correct 2. I and III only 3. I and IV only 4. II only Explanation: H 2 A and HA - or HA - and A 2 - will Form bu²ers, but H 2 A and A 2 - will not Form a bu²er. ±or polyprotic acids, protons become harder to remove (dissociate) as the ion be- comes more charged. ThereFore K a1 > K a2 For this system. In acidic solution (access pro- tons) all ions will be protonated. HA - is amphiprotic because it can accept a proton or it can donate a proton. 004 6.0 points The titration curve For the titration oF 0 . 5 M Na 2 CO 3 (aq) with 0 . 5 M HClO 4 (aq) is given below.

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Version 106 – Make up 2 – laude – (51635) 2 0 2 4 6 8 10 12 14 0 20 40 60 80 100 120 140 Volume of acid (mL) pH What are the main species in the solution after the addition of 35 mL of HClO 4 ? 1. HCO - 3 , Na + , and ClO - 4 . 2. CO 2 - 3 , Na + , and ClO - 4 . 3. CO 2 - 3 , HCO 3 - , Na + , and ClO - 4 . cor- rect 4. H 2 CO 3 , Na + , and ClO - 4 . 5.
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## This note was uploaded on 09/17/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.

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Makeup Exam 2, Spring 2011 - Version 106 Make up 2...

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