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exam3solution_pdf - Version 132 Exam 3 Sutclie(52440 This...

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Version 132 – Exam 3 – Sutcliffe – (52440) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A solution is 0.01 M BaCl 2 and 0.02 M SrCl 2 . Which cation can be selectively precipitated first with a concentrated Na 2 SO 4 solution? K sp is 1 . 5 × 10 - 9 for BaSO 4 , and 7 . 6 × 10 - 7 for SrSO 4 . 1. Ba +2 correct 2. Sr +2 3. Both will precipitate at the same time. Explanation: Before addition of Na 2 SO 4 ... [Ba 2+ ] = 0 . 01 M [Sr 2+ ] = 0 . 02 M Cl - is a spectator ion to be ignored. Each of the two cation concentrations listed above have a corresponding concentration of SO 2 - 4 that will cause each of these cations to precipitate (shown as ’ ’) as the sulfate salt. We must calculate these two SO 2 - 4 concen- trations and note which of the two is smaller (since we’ll get to that one first as we increase the SO 2 - 4 concentration from zero). SrSO 4 Sr 2+ + SO 2 - 4 K sp = [Sr 2+ ] [SO 2 - 4 ] 7 . 6 × 10 - 7 = (0 . 02 M) [SO 2 - 4 ] [SO 2 - 4 ] = 3 . 8 × 10 - 5 M for SrSO 4 BaSO 4 Ba 2+ + SO 2 - 4 K sp = [Ba 2+ ] [SO 2 - 4 ] 1 . 5 × 10 - 9 = (0 . 01 M) [SO 2 - 4 ] [SO 2 - 4 ] = 1 . 5 × 10 - 8 M for BaSO 4 The concentration of sulfate ion that will cause BaSO 4 to precipitate is smaller than the concentration that will cause SrSO 4 to precipitate. Therefore, Ba +2 will be precipi- tated first. 002 10.0 points Choose the effective pH range of a pyridine- pyridinium chloride buffer. For pyridine, the value of K b is 1 . 8 × 10 - 9 . 1. 7.7 to 9.7 2. 1.4 to 3.4 3. 10.3 to 12.3 4. 9.1 to 11.1 5. 4.3 to 6.3 correct Explanation: 003 10.0 points Adding NH 4 IO to pure water would result in what kind of solution? K a for HIO is 2 . 3 × 10 - 11 and K b for NH 3 is 1 . 8 × 10 - 5 . 1. acidic 2. basic correct 3. neutral Explanation: 004 10.0 points NOTE: These ’ionization constants’ are K a values. The ionization constants for an imagi- nary weak acid H 3 A are K 1 = 1 . 0 × 10 - 3 , K 2 = 1 . 0 × 10 - 7 , and K 3 = 1 . 0 × 10 - 11 . Would a water solution of NaH 2 A be acidic, basic, or neutral? Hint: Write the chemical equation and the equilibrium constant for the two possible reactions. 1. acidic because of further ionization of H 2 A - correct 2. basic because of the further ionization of H 2 A - 3. neutral 4. basic because of the hydrolysis of H 2 A -
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Version 132 – Exam 3 – Sutcliffe – (52440) 2 5. acidic because of the hydrolysis of H 2 A - Explanation: H 2 A - H + + HA 2 - K 2 = 10 - 7 H 2 A - + H 2 O H 3 A + OH - K b3 = K w K a1 = 10 - 14 10 - 2 = 10 - 11 005 10.0 points Which of the following acid-base indicators should be used for the titration of CH 3 COOH with KOH? For acetic acid, K a = 1 . 8 × 10 - 5 . 1. Methyl red, color change red/yellow at 4 . 4 < pH < 6 . 2 2. Any of these is suitable. 3. Neutral red, color change red/yellow 6 . 8 < pH < 8 . 0 4. Phenolphthalein, color change colorless/red-violet 8 . 0 < pH < 10 . 0 correct 5. None of these is suitable. Explanation: The endpoint of the weak acid (CH 3 COOH) - strong base (KOH) titration is above 7. An appropriate indicator has color change range pH = p K a ± 1.
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