Chem2 hw8s - Yoon, Seong Meen – Homework 8 – Due: Nov 5...

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Yoon, Seong Meen – Homework 8 – Due: Nov 5 2007, 11:00 pm – Inst: James Holcombe 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. The due time is Central time. 001 (part 1 oF 1) 10 points The solubility product constant oF PbCl 2 is 1 . 7 × 10 - 5 . What is the maximum concentra- tion oF Pb 2+ that can be in ocean water that contains 0.0500 M NaCl? 1. 1 . 7 × 10 - 3 M 2. 4 . 2 × 10 - 8 M 3. 8 . 5 × 10 - 7 M 4. 6 . 8 × 10 - 3 M correct 5. 3 . 4 × 10 - 3 M Explanation: K sp oF PbCl 2 = 1.7 × 10 - 5 PbCl 2 Pb 2+ + 2±Cl - K sp = [Pb 2+ ][Cl - ] 2 1 . 7 × 10 - 5 = [Pb 2+ ](0 . 05) 2 [Pb 2+ ] = 0 . 0068 002 (part 1 oF 1) 10 points Calculate the solubility oF iron(III) hydrox- ide at pH = 8 . 4. The solubility product oF iron(III) hydroxide is 2 × 10 - 39 . Correct answer: 1 . 26191 × 10 - 22 . Explanation: K sp = 2 × 10 - 39 pH = 8 . 4 Let S = molar solubility pOH = 14 - pH = 14 - 8 . 4 = 5 . 6 [OH - ] = 10 - pOH = 2 . 51189 × 10 - 6 The reaction is ±e 3+ (aq) + 3OH - (aq) * ) ±e(OH) 3 (s) K sp = [±e 3+ ][OH - ] 3 S = K sp [OH - ] 3 = 2 × 10 - 39 (2 . 51189 × 10 - 6 ) 3 = 1 . 26191 × 10 - 22 003 (part 1 oF 1) 10 points The mineral strontianite (SrCO 3 ) is quite in- soluble in water. The best reagent which could be added to dissolve a sample oF stron- tianite is 1. NaOH solution. 2. NH 3 solution. 3. H 2 CO 3 solution. 4. HCl solution. correct 5. SrCl 2 solution. Explanation: SrCO 3 -→ Sr 2+ + CO 2 - 3 To dissolve SrCO 3 , we need to shiFt the equilibrium oF this reaction to the right. We can do this by decreasing the amount oF CO 2 - 3 . We can do that by combining the CO 2 - 3 ion with H + to Form the weak acid H 2 CO 3 . This decrease in CO 2 - 3 will shiFt the equilibirum to the right causing the SrCO 3 to dissolve. OF the reagents listed, HCl and H 2 CO 3 are acids and can provide the H + ion. However, H 2 CO 3 will not only add H + , but also CO 2+ 3 and cause the equilibrium to shiFt back to the leFt. 004 (part 1 oF 1) 10 points Consider the Following K sp values For metal sulfdes: CdS = 3 . 6 × 10 - 29 CoS = 5 . 9 × 10 - 21 CuS = 8 . 7 × 10 - 36 PbS = 8 . 4 × 10 - 28 Which pair would be most di²cult to sepa- rate by Fractional precipitation?
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Yoon, Seong Meen – Homework 8 – Due: Nov 5 2007, 11:00 pm – Inst: James Holcombe 2 1. Cu +2 and Pb 2+ 2. Cd +2 and Co +2 3. Co +2 and Pb 2+ 4. Cd +2 and Pb 2+ correct Explanation: The pair that will be most difcult to sep- arate will be the pair with the most similar K sp values. These are CdS = 3 . 6 × 10 - 29 and PbS = 8 . 4 × 10 - 28 . This means that they will precipitate at about the same S 2 - concentra- tion. 005 (part 1 oF 1) 10 points IF you have 250 mL oF a solution that is 0.20 M [Ba 2+ ], how many grams oF Na±(s) should you add to reduce the equilibrium concentration [Ba 2+ ] to0.0001 M? Ba± 2 (s)is slightly soluble in water at 25 C, K sp = 4 . 7 × 10 - 6 .
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This note was uploaded on 05/14/2009 for the course CH 53575 taught by Professor Hackert during the Spring '09 term at University of Texas at Austin.

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Chem2 hw8s - Yoon, Seong Meen – Homework 8 – Due: Nov 5...

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