Exam%203%20Key%20F2007

Exam%203%20Key%20F2007 - n‘CHEleIISTFt"!r 241 EXAM...

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Unformatted text preview: n‘CHEleIISTFt"!r 241 EXAM III FALL Zflt'i?" Harrie: ' A Date: ' Show all yeur wet-It and elearl'j,r state any assumptiens yen make in a prehlem! ' Ne credit will be given fer sin-ind},r prewrting an anSWEI. * You must clearly,r shew year set-up and work. ' Ge threugh the exam and werk these prehlerns yeu knew hew te de first1 then start en the mere difficult preblerns. * An equatien sheet and periedie table have been peevrded at the end ef this exam * Use the space previded DO NOT use the hack 01" the exam. PLEDGE: Ihave net given ner renewed aid en the exam, ItUr have I seen anyene else give er receive aid en this exam. Signature: 59* Each \ée’ 1. (15 points) Indicate whether the followin statements are true (T) or false (F): There is only one angle that will result in total internal reflection. F If you double the frequency of electromagnetic radiation, you double the wavenumber. Total internal reflection can never be achieved if light is brought from a low refractive index medium (m) to a high refractive index medium (n2). In atomic (absorbance, fluorescence, or emission) spectroscopy the monochromator is placed before the flame. If you double the intensity of the incident radiation you double the absorbance in an absorbance measurement. T— :_ F— F". 2. (5 points) If the signal from a photomultiplier tube during a fluorescence experiment is 6.0 microamps and the noise of this tube is 3.2 microamps, what would the signal-to- noise ratio be if 5 measurements were averaged? 'm : C(‘ld Querqbtng (\ SPCCTQ , N 0‘7 fl M€95uffnxerfl3l S _ (9.0 m ,. /I\\’ “l Mubi £31:on L3G.qu 3. (8 points) Co pare and contrast fluorescence and phosphorescence. This question will be graded on thoroughness and completeness. l. Eo’th rqdwixmql Pfl‘aCz/JAES (Ewe off/and ljkll Cofld say be\\’\ Cm: iypw of \omnescpngg 1. Fluosesomoz \3 Sn LA in 30 id {3-35) and has orcxmuz \S lflpkl 95~053l<lb<7a3y U P Yb 3. Energ) kaeacpnq >Enegy/ o( PhoSphereS-quuz ‘l. WW3P O‘C3W"U OCCUSS S-l longer (\3 Hum FlQoH'JCQQCQ, S Lchkmc. of (PHOSleR'XCe-rxce 15 lent)” deCQUQQ 4. (6 points) Define the following types of noise and state whether they are dependent or independent on frequency. a.1/fnoise D\('\ O( ¥\I.L\'\cr “Gust; laleWULfl‘l' Dm‘lf‘lC Shaw Chgotges \(‘\ \OSMNWNW Phat/tans Q? rm em? 6" angina; tk‘ \ujlfi ; FCC cfisené‘y D: pgncl‘afi'l- J lZPCQ Ul EflmmHMrfl‘l‘fi‘l NQHSQ QM\L¥\ OCLVFS q syeeubc @ccbsancxes éDePendm’l ofl Ftectoeno/ b. Line Noise K 62/ 5. (6 points) 10.1 mg of a substance with a olecular weight of 255 g/mol was dissolved in 10.0 mL of water in a volumetric flask. 4.0 mL of this solution was transferred to a cuvet with a 1.00 cm path length and the transmittance was measured to be 34.9 % at a wavelength of 503 nm. Calculate the molar absorptivity for this substance at 503 nm with the correct units. t e L (MA Cm_\ Pv- —-L053'T = 4.03 0.34%: ,3 . lO.'\X\O mo\ x __ _ ,53 3X255j 0.0mm. ‘ 3%)“) M O . 4 57 z e (i-a3cm\(8%>< 0’34) ,1“ Cloned ninth ’Ry' mo\cr CtbSKIVN/t‘k/v + 4 a. The bandwidth of light from the light source in atomic absorption spectroscopy should have a as compared to the absorption band being studied. Circle choice. ( i. smaller bandwidth 2 ii. same bandwidth iii. larger bandwidth b. In the context of atomic spectroscopy, the Heisenberg uncertainty principle says that . Circle choice. 6. (25 points) Answer the following questions. i. the shorter the lifetime of the excited state t - - I ain is its ener ii. the longer the lifetime of the excited state, the more uncertain is its energy. iii. The Heisenberg uncertainty principle says nothing about the lifetime of the excited state and uncertainty in energy levels. 4” 5 c. (5 points) What does the acronym LASER stand for? +‘ {3r eth bmp‘cfic‘thm filled E pmgg . on 35%).qu H d. Which light source is used if the desired wavelengths are in the UV region of the + electromagnetic spectrum? Circle Choice. i. Tungsten Filament ii. Globar ( iii. Deuterium Arc Lamfi’ iv. Not Listed k0 e. In atomic absorption spectroscopy the follo ng type of light source is used? i. Laser ii. Deuterium arc lamp iii. Tungsten filament iv. No 1i ht source is used in atomic absorption spectroscopy <v. Eollow cathode lamfi) + 4 f. The region of the electromagnetic spectrum that is best suited to observe vibrational transitions? ’ "“ iii- X-Ray iv. UV—Vis 7. (10 points) Describe how a 1as§§$§M$hmfl\g l. RAMP-.03 ‘ \MPUV’i mi lands) ’2“ C(e 9““: 909A ‘tbcfl ereercfl '3. Sixmulq‘lcd Eats‘sm - destabea . :1: Qfiggcckagncsxsdel of phoitmos. Es \ @ EL 7 Q} E \‘l/ “‘9qu “AH Also a L 1:o 8. (15 points) You are an EPA scientist in charge of monitoring the cooling water effluent from a local manufacturing plant. The manufacturing plant uses compounds containing hexavalent chromium to manufacture paint pigments. Hexavalent chromium, or Cr (V1), is known to cause lung cancer when inhaled, although debates persist whether it could cause cancer when consumed orally as claimed in the movie, “Erin Brokovich”. You monitor the cooling water effluent daily for its Cr (VI) concentration. Your routine method for daily monitoring of Cr (VI) in the cooling water effluent is as follows. You collect a 100.0 mL sample from the cooling water effluent from the storage tank at the plant. You add 25.0 mL of a reagent that will react specifically with Cr (VI) to form a colored product. The molar absorptivity of the colored Cr (VI) complex is 9890 M'1 cm'1 at 400 nm. On 11/5/07 you collected your cooling water effluent sample as usual. After adding the complexing reagent to the sample, the resulting solution was very strongly colored. You diluted a 2.00 mL aliquot of the colored solution with deionized water to a total volume of 20.0 mL. You place 1.00 mL of this weakly colored solution in a 1.00 cm path length cell. This sample exhibited an absorbance of 0.225 at 400 nm. Your reference blank kg solution exhbited an absorbance of 0.085. Whi is the concentration, in M, of Cr (VI) in the cooling water effluent sample you collected on 11/5/07? Ax - Abany, 1 ébCfl dink 0.11 5 —o . 0%5: (qQfio MACM‘XxOQCmyfl (133C [flaunt {Auto'f’m 0.41 K‘b’SMXZQOmQ 2 [X'Lcmdmd‘zoomg % CM, ': C.va E70 cumth ’— |.41>< 30—4 M (t./1’l><)o'4w0(\2.5’m0 1 [flaunt (lo-amt) 9. (10 points) The burning of magnesium, iron and other metals results in the brilliant displays observed during the 4th of July (Fireworks). If the energy of photons emitted by excited Mg atoms is 3.50 x 10'19 J, what is the wavelength and wavenumber, with the appropriate units, of such ener y. 2 W; E 5 ex . 3.4L . 0 q, . ® LO‘RWiem-‘HW‘ 3 S. , Ji— <<OIQ2Q7700 J'SBLZFHQJXIO “7/5) K) ' t J : O <7; mqwnumbe’r (98‘ (:2: ‘i/fx \fi CmJ " fl £08m“ x I,O;<\O m mom X L Cm V: 5943x656”, : Wcosccm“ (i ncmoim") \WMS} bfi \fl C(fl-l m ...
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This note was uploaded on 03/21/2008 for the course CHEM 261 taught by Professor Austell during the Spring '08 term at UNC.

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Exam%203%20Key%20F2007 - n‘CHEleIISTFt"!r 241 EXAM...

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