Chem 321: Exam 3 - Fall 2009 - Prof. Stephen F. Wolf, Exams of Analytical Chemistry

Download Chem 321: Exam 3 - Fall 2009 - Prof. Stephen F. Wolf and more Exams Analytical Chemistry in PDF only on Docsity! Chem 321: Exam 3 Fall 2009 1 EXAM 3 Chem 321 November 23, 2009 Name: _______________________________________ Student ID: ___________________________________ Show all pertinent work for credit. Report results with correct units and significant digits. Be sure to check any simplifying assumptions that you may make. All problems are for 298 K. 1. Consider the titration of 10.0 mL of 0.250 M ClCH2COOH (chloroacetic acid) with 0.100 M NaOH. For chloroacetic acid Ka=1.36x10-3. Calculate the answer to three decimal places. 1a. (15 pts) Calculate the initial pH of the 0.250 M ClCH2COOH solution. Answer: Chem 321: Exam 3 Fall 2009 2 1. Contd. Consider the titration of 10.0 mL of 0.250 M ClCH2COOH (chloroacetic acid) with 0.100 M NaOH. For chloroacetic acid Ka=1.36x10-3. Calculate the answer to three decimal places. 1b. (20 pts) Calculate the pH of solution after the addition of 15.0 mL of 0.100 M NaOH. Answer: Chem 321: Exam 3 Fall 2009 5 4. (10 pts) Consider the following (unbalanced) redox reaction. E°UO22+/U4+=+0.334 V and E°Zn2+/Zn=-0.763 V. UO22+ + Zn  U4+ + Zn2+ a. Write both the reduction and oxidation half reactions and the balanced net reaction. b. Calculate the thermodynamic standard free energy for this reaction at 298 K. Answer: 5. (6 pts) Select or write the best answer in the space provided. Consider the following standard electrode reduction reactions: (i) O2 + 4H+ + 2e-  2H2O E°= +1.23 V (ii) Fe3+ + e-  Fe2+ E°= +0.771 V (iii) HClO + H+ + e-  0.5 Cl2 + H2O E°= +1.63 V a. Which of the following is the strongest oxidizing agent? A) Fe2+ B) HClO C) O2 D) Cl2 E) Fe3+ Answer: __________ b. Which of the following is the strongest reducing agent? A) Fe2+ B) HClO C) O2 D) Cl2 E) Fe3+ Answer: __________ Chem 321: Exam 3 Fall 2009 6 6. (9 pts) When a voltmeter is connected to the following electrochemical cell a potential of -0.77 V is measured. Pt | Fe3+ (0.001 M), Fe2+ (0.001 M) || SHE If you short-circuited the cell would the electrode on the left act as an anode or cathode? Answer: _________________________ If you short-circuited the cell would the above electrode on the left be the site of an oxidation or reduction reaction? Answer: _________________________ If you increase the concentration of Fe2+, which of the following statements is true about the cell potential? A) It would increase. B) It would decrease. C) It would remain constant. D) Cannot be determined from the given information. E) None of these (a-d). Answer: ___________ BONUS. What is the pH of 1.00x10-8 M HCl? Calculate the answer to three decimal places. Answer: CONSTANTS: Kw=1.00x10-14; R = 8.314 J/mol K ; F = 96485 C/mol. EQUATIONS: ! "G = #nFEcell "G o = #nFEcell o "Go = #RT lnKeq Ecell o = ER o #EL o Ecell = ER #EL lnKeq = abF ER o #EL o( ) RT ! E = Eo " RT nF ln C[ ] c D[ ] d A[ ] a B[ ] b [H3O + ] = Ka2 cHA- + Kw 1+ c HA- Ka1( ) x = "b + b 2 "4ac 2a