Exam_2_2010_answer_2_key - Problem # 1 (16 pts): Fractional...

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Problem # 1 (16 pts): Fractional Composition Diagrams A fractional composition diagram is plotted (below) for o-phosphoric acid. (a) (4 pts) Write the pertinent acid-dissociation reaction equations in the full pH range. H 3 PO 4 + H 2 O ↔ H 3 O + + H 2 PO 4 - H 2 PO 4 - + H 2 O ↔ H 3 O + + HPO 4 2- HPO 4 2- + H 2 O ↔ H 3 O + + PO 4 3- (b) (4 pts) Give the approximate K a values (not pKa’s) as read from the plot. 10 -2.15 = 7.1 x 10 -3 These are the actual values. 10 -7.20 = 6.3 x 10 -8 10 -12.15 = 7.1 x 10 -13 (c) (4 pts) What is the principal species at pH = 14.00? PO 4 3- or A 3- (d) (4 pts) What is the principal species at pH = 0.00? H 3 PO 4 or H 3 A pH 0 2 4 6 8 10 12 14 α (fraction of each form) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Fractional composition diagram for o-phosphoric acid
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Problem # 2 (18 pts): Redox Titrations 100 mL of 0.100 M Fe 2+ was titrated with 0.100 M Ce 4+ in 1.00 M HNO 3 . Formal potentials for Fe 3+ /Fe 2+ and Ce 4+ /Ce 3+ (in 1.00 M HNO 3 ) are 0.746 V and 1.61 V. (1) (6 pts) What was the potential at the equivalence point? Derive the expression you use to calculate this value from Nernst equations. (If you can’t derive it, you will get partial credit for the short-cut equation.) E eq = ½( E Ce o + E Fe o ) = ½ (1.61 + 0.746 ) = 1.18 (2) (6 pts)What was the equilibrium concentration of Ce
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This note was uploaded on 12/28/2010 for the course CHEMISTRY 222 taught by Professor Wieckowski during the Spring '10 term at University of Illinois at Urbana–Champaign.

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Exam_2_2010_answer_2_key - Problem # 1 (16 pts): Fractional...

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