Chem 310, Fall 2005 Final Exam December 15, :15-12:15 (You must stop working at 12:15!)

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1 NAME: Chem 310, Fall 2005 Final Exam December 15, :15-12:15 (You must stop working at 12:15!) There are 3 pages of questions on this exam (not counting this page), with points allocated as indicated. You should use the points to gauge the division of your effort. Liberal partial credit will be given for correct set-up; be sure to show all work in your exam booklet. Please write clearly and try to organize your answers to make them easy to follow! Remember to write your name clearly on the exam book cover and on these questions; both will be collected. You may use your calculator and a single 5" 8" formula card (written on both sides). Any necessary tabulated data is provided. Assume all solutions are aqueous unless stated otherwise. Grades will be posted by Monday afternoon. If you would like your final exam score and course grade posted, along with the last 4 digits of your social security number, complete and sign the statement below: I hereby request that my final exam score and course grade for Chemistry 310 for Fall, 2005, be posted, along with the last 4 digits of my social security number. ( ) Insert last 4 digits here Signature

2 Chem 310 Fall, 2005 Final Exam 29 pts. 1. Consider the electrochemistry of the dye "SNARF" (there really is a dye by this name!): 2 SNARF + 2 e 2 {SNARF-H} + H 2 E ~ V ({SNARF-H} is the deprotonated form of the dye.) a) For the cell below, which electrode is the cathode and which is the anode? Pt H2 (g, 1 atm.), HCl (a = 1.0 F) SNARF, HCl (a = 1.0 mf), H2 (g, 1 atm.) Pd 5 pts. b) What does the double vertical line ( ) represent? Can it affect the cell potential? If so, how, and with what consequences? If not, why not? 3 pts. c) What is the potential of the half-cell containing 1 F HCl? 3 pts. d) Write a Nernst equation for the half-cell containing SNARF. 9 pts. e) If Ecell = V at room temperature, what is the concentration ratio [{SNARF-H} ]/[SNARF]? 7 pts. f) Using your answer from part (e), estimate K a for SNARF. (HINT: Other necessary information is provided with the cell description). 27 pts. 2. Short answers dealing with chromatography: 5 pts. a) In gas chromatography, it is common to use temperature programming to affect the equilibration of analytes between the stationary and mobile phases. By this approach, materials with successively greater affinity for the stationary phase can be eluted in turn. Is it more common to increase the temperature during a temperature program, or to decrease it? Why? 8 pts. b) In liquid chromatography, it is generally the composition of the mobile phase which is programmed (varied) during an elution, rather than the temperature. The LC experiment is then said to employ "gradient elution." Explain why temperature programming is uncommon in LC, and why "gradient elution" is uncommon in GC. 5 pts. c) Consider ion exchange chromatography. What sort of gradient is most likely to be useful in this experiment? Explain, based on the principles of ion exchange. 5 pts. d) H (or HETP "height equivalent to a theoretical plate") is a common measure of the quality of a chromatographic column. It is generally smaller (by roughly an order of magnitude) in LC than in GC. Nevertheless, column performance tends to be fairly similar in these two major branches of chromatography. Explain how this can be true. 4 pts. e) Given that column performance in LC and GC tend to similar (see part (d), above), and the fact that materials are more likely to be soluble (and therefore compatible with LC) than they are to be volatile (a requirement for GC), why not save on equipment costs and do all chromatographic experiments with a liquid mobile phase?

3 2 17 pts. 3. The distribution ratio (D K D ) for arsenic (III) between benzene (a non-polar organic solvent) and 7 F HCl is 2.33: D = F F As, benzene As, 7F HCl = pts. 77 The pts. a) What volume of benzene would be necessary to remove 85% of the arsenic(iii) from 100 ml of a 7 F HCl solution in a single extraction? b) What is the likely chemical form of arsenic(iii) in the benzene layer? (Hint: Consider the purpose of the HCl). 4. The amount of iron (AW ) in a "Forever Young" vitamin supplement was measured by burning the organic material under conditions where all iron is converted to Fe(III), then assessing the iron concentration from the absorbance of its 1:1 complex with ligand "Q", using 1.00 cm cells and assuming that Beer's Law applies. A 4.60-g vitamin tablet was burned and the sample residue (including all of the iron) was diluted to 2.00 L in a volumetric flask. Aliquots of the resulting solution were treated with excess Q, with or without addition of an iron standard and/or water, as indicated in the table below. A pure water blank was also prepared. Measurements of transmitted power (P) were then made using a primitive spectrometer (without a computer to convert P to A): Solution # Added volume (ml) Sample ppm Fe(III) standard Ligand Water P pts. 5 pts. 10 pts. 10 pts. a) What is the purpose of preparing and measuring solution #2? b) Calculate the % transmittance (%T) and the absorbance (A) of solutions #3 and #4. c) The procedure outlined above is an example of one of the three approaches to spectrophotometric quantitation discussed in class. Name the approach. (HINT: This is NOT a 1-point calibration!) d) Are the absorbances calculated in part (b) within the optimum range for accurate absorbance measurements? Explain briefly why higher or lower absorbances are difficult to measure. e) Calculate the molar absorptivity (ε) of FeQ, assuming that it is the only species absorbing at the wavelength employed. Is this value within the range of "typical" values? (HINT: Since these are dilute aqueous solutions, you can assume their density is 1.00 g/ml.) f) Calculate the concentration of Fe(III) in the 2-L sample.

4 4 pts. g) Calculate the mass of iron in the tablet. 3 4 pts. 10 pts. h) "Forever Young" vitamins claim to provide 35 mg of iron in each tablet. Recall from lecture that the US Food and Drug Administration generally allows tolerances of ±25%, so to be "within specification," the tablet must contain between 27.5 and 42.5 mg of iron. From your answer to part (g) and the other information above, is it possible to determine whether the sampled tablet is "within specification" (i.e., there is less than a 5% chance that the experimental value lies outside FDA tolerance)? If so, show by calculation whether the tablet meets specifications. If not, tell what additional information is needed. i) The quality control staff at "Forever Young" has extensive experience with this spectrophotometric method for iron, from which they know that the standard deviation (σ) for assessing the mass of iron in a tablet is 0.2 mg. Does this additional information affect your conclusion concerning whether the tablet analyzed above is within specifications? If so, show by calculation whether the tablet is within specifications, according to the definition in part (h). If not, tell why not. j) An alternative approach for iron analysis might involve direct potentiometry. Would an iron metal electrode be suitable for such an analysis? Why or why not? k) Yet another option could be to do a potentiometric titration. i. From the table of redox half-reactions provided with this exam, select a suitable titrant for this analysis. 4 pts. 6 pts. 3 pts. ii. iii. iv. What would be suitable electrodes (indicating and reference) for this titration? Sketch the titration curve, indicating the approximate pre- and post-equivalence point potentials. Clearly label the axes in your sketch. For the x-axis, use "% Titration." Be sure to indicate whether your y-axis is E ind or E cell. You need not calculate the equivalence point potential (I'll give 2 points "extra credit" if you have time to do the calculation), but you should indicate approximate values for E at 50% titration and beyond the equivalence point. Could EDTA be used as a titrant for the potentiometric titration of Fe 3+? If so, could you use the same electrode system as in part (k(ii))? If not, why not? 9 pts. l) Compare the three methods discussed above (spectrophotometry, potentiometry, and potentiometric titration) in terms of their accuracy, precision, and sensitivity.

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6 Table 16-1 Standard Electrode Potentials Reaction E 0 at 25 C, V Cl 2 (g) + 2e 2Cl O 2 (g) + 4H + + 4e 2H 2 O Br 2 (aq) + 2e 2Br Br 2 (l) + 2e 2Br Ag + + e Ag(s) Fe 3+ + e Fe I 3 + 2e 3I Cu e Cu(s) UO H + + 2e U H 2 O Hg 2 Cl 2 (s) + 2e 2Hg(l) + 2Cl AgCl(s) + e Ag(s) + Cl Ag(S 2 O 3 ) e Ag(s) + 2S 2 O H + + 2e H 2 (g) AgI(s) + e Ag(s) + I PbSO 4 + 2e Pb(s) + SO Cd e Cd(s) Zn e Zn(s)