Final Exam. Physical Constants and Conversion Factors. Equations

Transcription

1 Final Exam Instructions: This exam is worth 100 points. Some questions allow a choice as to which parts are answered. Do not answer more parts than are requested. velocity of light in a vacuum: 3.0x10 8 m/sec Planck's constant: 6.626x10-34 Joule-sec electron charge: 1.602x10-19 Coulomb 1 Joule = 1 Volt-Coulomb Avogadro's constant: 6.022x10 23 atoms/mole 1 nm = 10-9 m 1 cm = 10-2 m?? = c E = h? E = ev sin r = n?/d - sin I dr/d? = n/(d cos r) sin I / sin r = n 2 / n 1 R =? /?? R = S / HA S = S final (1 - e -t/t ) A = ebc A = -log 10 T T = P/P 0 A total = A A n A = log 10 ((P 0 ' + P 0 ")/(P 0 '10 -e b + P 0 "10 -e bc )) A = -log 10 ((P + P s )/(P 0 + P s )) F = kp 0 (1-10 -ebc ) F? kp 0 (2.303)ebC I(d) = 0.5 I(?)(1 + cos(2pd/?) I(t) = B(?) + B(?)cos(2p?2Vt) f = 2?V I(d) =?B(?)cos(2p?d) d? B(?) =?I(d)cos(2p?d) dd Physical Constants and Conversion Factors Equations

2 1. (16 points) Answer two of the following. a. Why does an atomic absorption spectrometer typically operate at higher resolution than a corresponding spectrometer used in molecular UV-visible absorption spectroscopy? b. Name three advantages and three disadvantages of inductively coupled plasma atomic emission spectroscopy relative to flame atomic absorption spectroscopy. c. Describe the operating principles of a hollow cathode lamp. 2. (25 points) Answer five of the following. a. What is the most common light source used in near-infrared absorption spectroscopy? b. Of the detectors discussed in this course, which has the highest responsivity in the UV-visible region? c. Give an example of a wavelength of light (in nm) that would be absorbed by a blue solution. d. Name two factors that control the sensitivity of an absorbance measurement.

3 e. Name three ways to improve the resolution of a Czerny-Turner monochromator. f. For a signal measurement in Volts and a concentration measurement in mg/l, what are the corresponding units of sensitivity? 3. (24 points) For each of the following statements, indicate whether the statement is true or false and provide a statement that justifies your choice. a. An increase in source intensity will lead to a greater luminescence signal because the quantum yield is increased. b. An interferometer is required in a Fourier transform optical spectrometer in order to increase optical throughput and thereby overcome the limitations of weak infrared sources. c. Oxygen molecules (O 2 ) have no absorption bands in the infrared region because the O-O bond strength is too high.

4 4. (20 points) You have the following instrumental configurations: A. Double-beam luminescence spectrometer with Xe arc lamp and photomultiplier tube detectors. B. Double-beam luminescence spectrometer with Ar + laser and photomultiplier tube detectors. C. Fourier transform infrared spectrometer. D. Double-beam UV-visible absorption spectrometer with tungsten-halogen source and phototube detector. E. Single-beam UV-visible absorption spectrometer with tungsten-halogen source and photodiode array detector. F. Double-beam UV-visible absorption spectrometer with tungsten-halogen source and photomultiplier tube detector. G. Inductively coupled plasma atomic emission spectrometer. H. Flame atomic absorption spectrometer. I. Furnace atomic absorption spectrometer. For each of the following measurement scenarios, indicate the best instrument configuration to use by specifying the appropriate letter from the list above. a. You are analyzing Cu, Pb, and Zn in a water sample. b. You wish to measure polycyclic aromatic compounds with the lowest possible limit of detection. c. You wish to measure compounds with known chromophores eluting from a chromatography column. d. You wish to differentiate pentane (CH 3 (CH 2 ) 3 CH 3 ) and hexane (CH 3 (CH 2 ) 4 CH 3 ). e. You are analyzing Mg in a steel sample in a setting in which minimizing the analysis time and analysis cost are of primary importance.

5 5. (7 points) The silver iodide bond energy is approximately 255 kj/mole. What is the longest wavelength of light (in nm) that is capable of breaking the bond in silver iodide? To what region of the electromagnetic spectrum does this wavelength correspond?

6 6. (8 points) Iron(II) ions catalyze the oxidation of luminol by H 2 O 2. The intensity of the resulting chemiluminescence has been show to increase linearly with iron(ii) concentration from to 10-8 M. A mg solid sample containing iron (AW = 55.85) was dissolved in acid and diluted to 50.0 ml in a volumetric flask. Exactly 1.00 ml of water was added to a 2.00 ml aliquot of the unknown iron solution, followed by 2.00 ml of a dilute H 2 O 2 solution and 1.00 ml of an alkaline solution of luminol. The chemiluminescence signal from the mixture was integrated over a 10 second period and found to be 16.1 µa. To a second 2.00 ml aliquot of the unknown iron solution was added 1.00 ml of a 4.75x10-5 M iron(ii) solution followed by the same volumes of H 2 O 2 and luminol used above. The integrated chemiluminescence signal was 29.6 µa. Compute the weight % of iron in the original solid sample.