Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !1! General Instructions: KEY Time: 9:05 – 9:55 Points: 15 (maximum points for the course is 100) Bonus points (self-study): 3 points Allowed: One-sided, 8.5” x 11” crib sheet for equations; one calculator. Not Allowed: See Syllabus Regrading: Considered only if the exam was written in ink. 1. State in two sentences your analytical problem. [0.5 points]. Answers vary. 2. For each criterion (first row, table below), select with a check mark (√) the technique that is best suited to meet the demands of your analytical problem. Provide an explanation in the next page. [2.5 points] Answers vary. Below, you will find some general guidelines. 1=Best, 9=worst. Each criterion is worth 0.4 points when a logical explanation was provided in the following page. If no explanation was provided, no points were given. It is expected that this information will later be confirmed with literature sources. There will be bonus points assessing precision and accuracy in the final exam. Techniques Criterion UV-VIS absorption Fluorescence IR AAS AES MS Voltammetry Selectivity 9 1 1 1 1 1 1 LOD 9 1 4 6 2 1 Atomic or molecular characterization capabilities 9 1 1 1 1 5 Precision 1 1 5 2 2 7 2 Accuracy 2 2 5 2 2 7 2 Overall best suitedChem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !2!Provide a very short explanation for each parameter Selectivity: Fluorescence is best when only the analyte is fluorescent. IR and MS are adequate when the analyte is pure. MS is also good for mixtures if MS/MS provides confirmation. Voltammetry is excellent when only the analyte is electroactive. For atomic species with unique spectral lines AAS or AES are good techniques. LOD: Fluorescence and electrochemical techniques are known for their superior LOD’s. MS is good but not all the analyte is converted to ions. AAS and AES have similar limitations and usually have more background noise; thus less impressive S/N, i.e., LOD’s. Atomic or molecular characterization capabilities: See the explanations given for selectivity. They apply to this criterion. Precision: Techniques with stable sources and detectors, large sample volumes or long measurement times provide the best reproducibility. These techniques include UV-VIS absorption, fluorescence, AES, AAS. This is not true for ETA. IR may also have good precision when FT-IR is used. MS has terrible reproducibility due to instability in the electrospray process, chemical ionization, or MALDI. Accuracy: See the explanations given for precision. The main issue here is the nature of the sample matrix and the ability to use methods that can account for matrix effects. Overall best suited technique:Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !3! 3. The compound fluorescein is fluorescent. See absorption and emission spectra on the right. Draw a hypothetical energy diagram (Jablonski Diagram). In this diagram indicate transitions that correspond to the absorption and emission spectra. Also, indicate at the bottom of each relevant transition, the wavelengths of maximum emission or absorption. [1.5 points] 4. List two reasons why fluorescence spectrometry is more sensitive than UV-VIS absorption spectrometry. [0.5 points; Problem 15-3 and lecture notes]Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !4!5. Draw two linear voltammograms for Cr2+/ Cr3+in the same plot. Consider concentrations and potentials in your sketch. Label the axes. Eo = -0.408 V; Reference electrode: SCE; EoSCE = 0.25 V. [1.5 points] First voltammogram: Assume that the solution contains only 1x10-3 M Cr3+. Second voltammogram: Assume that the solution contains only 1x10-3 M Cr2+. (a) Identify the limiting currents. Identify in the plot and provide the value for the half-wave potential. (b) For each voltammogram, indicate whether it corresponds to either a cathodic or anodic wave. (c) For each voltammogram, write the electrochemical reaction that leads to the observed waves.Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !5!6. Consider the following electrochemical cell: Pt|Fe3+ (4.00x10-2 M), Fe+2 (3.00x10-5 M) || Sn2+ (5.50x10-2 M), Sn+4 (3.5x10-4 M)|Pt Standard electrode potentials: For Fe3+/Fe2+, E° = 0.771V For Sn4+/Sn+2, E° = 0.154 V Using this nomenclature, one vertical line represents an interface, two vertical lines represent a salt bridge and the direction of the reactions is always from left to right. [1.5 points. Problem 22-7, part c, and example 22-8] (a) Calculate the electrochemical potential of this cell. (b) Is this a galvanic cell as written? Explain. (c) If this cell has an internal resistance of 4.0 Ω and initially shows a current of 0.10 A, what potential is produced (if it is a galvanic cell) or needed (if it is an electrolytic cell)?Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !6! 7. List three advantages of an FTIR spectrometer compared with a dispersive instrument? [0.9 points. Problem 16.8] 8. An interferogram (time domain) determined using a Michelson interferometer is described by the equation: P(δ) = B1(€ ν1)cos(2πδ€ ν1) + B2(€ ν2)cos(2πδ€ ν2) where € ν2 = 2€ ν1 Draw one spectrum (Fourier transform) in which the intensities at € ν2 and € ν1 are in a 2:1 ratio. [1.1 point]Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !7! 9. In flame atomic absorption spectrometry (flame AAS), what processes occur in the flame? Which ones are irreversible? Answer by filling the table. [0.8 points] Flame process Irreversible Ionization Desolvation Nebulization Molecular dissociation 10. Why are atomic emission methods with an ICP source better suited for multi-element analysis than are flame atomic absorption methods? [1.2 points. Problem 10-2]Chem 4101 – Fall 2010 EXAM 1 November 12, 2010 Write your name in the last page only ! !8! 11. The natural abundances of 37Cl and 13C relative to 35Cl and 12C are: 1% and 32%, respectively. Calculate the relative intensity of the two isotopic peaks with m/z values closest to (12C35Cl4)+. Draw the molecular ion spectrum that includes the first three isotopic peaks (i.e., the peak for (12C35Cl4)+ and
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