CHE 226 ANALYTICAL CHEMISTRY Fall 2005 EXAM III – December 1, 2005 Name ______________________________ WRITE YOUR NAME ON EACH EXAM PAGE NOW. THERE ARE 5 QUESTIONS AND 104 PERCENT TOTAL IN THIS EXAM. Show clearly all work on these pages. Use the proper number of significant figures and the correct units in all final answers. You must show your calculations and/or reasoning, including equations, on a question to obtain any credit; no credit for answers appearing out of the blue. Your work must be understandable at the time it is being graded to obtain any partial credit. You do not have to do the final arithmetic on a question unless you need to have a numerical value for the next part of a question, as long as the answer is expressed in its final form and all algebraic manipulations have been made. Very little will be subtracted for routine arithmetic errors, but all numerical answers must be shown to the proper number of significant figures. Programmable calculators must have all memory erased. A calculator may be used, but not shared with anyone else. Tables of data and other information that may be useful are appended to the back of the exam. Use the backs of the pages as scrap paper. Anything written on the backs of pages is totally irrelevant to the grading process. Unless otherwise stated, assume all solutions are aqueous, density = 1.0000 g/mL; activity coefficients are unity (i.e., activity = concentration); temperature, T = 298 K; Kw = 1.008 x 10-14. QUESTION 1 ___________ /24 Question 7 ___________ / QUESTION 2 ___________ /27 Question 8 ___________ / QUESTION 3 ___________ /15 Question 9 ___________ / QUESTION 4 ___________ /20 Question 10 ___________ / QUESTION 5 ___________ /18 Question 11 ___________ / QUESTION 6 ___________ / TOTAL ___________ /104Name __________________________________ CHE 226 Exam III Fall 2005 21. (24 points) Calculate the pH of the solution that results when 20.0 mL of 0.200 M formic acid (HCOOH, Ka = 1.80 x 10−4) is (a) Diluted to 45.0 mL with distilled water. (b) Mixed with 25.0 mL of 0.200 M NaOH solution. (c) Mixed with 25.0 mL of 0.200 M sodium formate solution.Name __________________________________ CHE 226 Exam III Fall 2005 32. (27 Points) Malic acid is a diprotic acid. Designate it as H2M for simplicity. Its acid dissociation constants are 3.40 x 10−4 and 8.00 x 10−6. Calculate the theoretical pH of the following solutions. (a) 0.20 M malic acid, H2M (b) 0.20 M monosodium malate, NaHM. (c) 0.20 M disodium malate, Na2M.Name __________________________________ CHE 226 Exam III Fall 2005 43. (15 Points) Tris(hydroxymethyl)aminomethane, often called TRIS or THAM, buffers are used quite commonly by biochemists because they have several desirable properties. TRIS itself is the basic form, but the acid form (pKa = 8.08) is commonly sold as the stable HCl salt. You wish to prepare a TRIS stock buffer solution of approximate physiological pH (7.40), with a total buffer concentration (acidic + basic forms) of 0.100 M. How many moles of the protonated acidic form, designate it as HT+ for simplicity, and of the basic form T would you need to weigh out in order to prepare 1.00 L of such a buffer? 4. (20 points) The ferric ion, Fe3+, can be complexed by up to three chloride ions to form soluble complexes. The three stepwise formational constants have been measured to be Kf1 = 30, Kf2 = 135, and Kf3 = 98. You have a very low concentration of the ferric ion, say about 10−4 M, in a solution of 0.100 M KCl. Under these conditions, which iron species, the free ferric ion or one of its complexes, is the predominant form, and what fraction (α) of the total iron concentration does it comprise? [Recall that you do not have to do the final arithmetic, but would need to do some intermediate arithmetic to answer the first question posed.]Name __________________________________ CHE 226 Exam III Fall 2005 55. (18 Points) Consider the fractional distribution diagram (α plot) for an unknown acid, HnA, below. (a) How many ionizable, acidic protons does this acid have in the pH range shown? How did you conclude this? Carefully label each line on the plot with its appropriate αn. (b) Estimate the pKa value(s) for this acid. Show on the plot how you arrived at the estimate(s). (c) Estimate the fractional abundance, α, of each of the various forms present at pH 9. 0.00.10.20.30.40.50.60.70.80.91.04 6 8 10 12 14pHαName __________________________________ CHE 226 Exam III Fall 2005 6SELECTED CONSTANTS, UNITS, AND CONVERSION FACTORS [The uncertainty in the last digit(s) is shown italicized in parentheses] Atomic mass constant: mu = 1.660 538 73 x 10-27 kg Avogadro’s number: N = 6.022 141 99 (47) x 1023 mol-1 Boltzmann constant: k = 1.380 6503 (24) x 10-23 J/K Elementary charge: e = 1.602 176 462 (63) x 10-19 C Faraday constant: F = 96 485.3415 (39) C/mol Molar gas constant: R = 8.314 472 (15) J/K-mol = 1.9872 cal/K-mol = 0.082 057 L-atm/K-mol = 0.022 414 m3/mol at STP Pi: π = 3.141 592 653 6 Planck’s constant: h = 6.626 068 76 (52) x 10-34 J-s Speed of light (in a vacuum): c = 2.999 792 458 (exact) x 108 m/s Stefan-Boltzmann constant: σ = 5.670 400 (40) x 10-8 W/m2-K4 Standard acceleration of gravity: gn = 9.806 65 (exact) m/s2 Wein constant: k = 2.897 7686 (51) x 10-3 m-K Force: 1 N = 1 kg-m/s2 Joule: 1 J = 1 N-m = 1 kg-m2/s2 = 107 ergs = 1 V x 1 C = 1 V-C = (J/C)(C) Power: 1 W = 1 J/s = 1 V x 1 A = 1 V-A = (J/C)(C/s) Electron Volt: 1 eV = 1.602 176 462 (63) x 10-19 J = 3.827 x 10-20 cal Calorie (thermochemical): 1 cal = 4.184 J [Food “calorie” = 1 Cal = 1000 cal] Length: 1 km = 1000 m = 0.62137 mi 1 in = 2.54 cm (exactly) Mass: 1 kg = 1000 g 1 pound = 453.59237 g Pressure: 101 325 (exact) Pa = 1 atm = 760 mm Hg = 17.70 lb/in2 133.322 Pa = 1 torr = 1 mm Hg 105 Pa = 1 bar 1 Pa = 1 N/m2 Volume: 1 L = 10-3 m3 = 1000 mL = 1000 cm3 = 1.056710 quarts Nernst factor: (RT/nF) ln = (0.05916 V/n) log10 at 25 oC nF/RT = 38.920 n V-1 RT/nF =