midterm 2005 - Chemical Instrumentation CHEM*3440 Mid-Term...

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Chemical Instrumentation CHEM*3440 Mid-Term Examination Fall 2005 TUESDAY, OCTOBER 25, 2005 Duration: 2 hours. You may use a calculator. No additional aids will be necessary as a series of data and equation sheets are included as part of the examination paper. 1. (8 points) Polycyclic Aromatic Hydrocarbons (PAH) is a class of organic compounds that have important toxicological effects. Their detection and measurement in environmental conditions is a vital health concern. A method was developed to measure these compounds from waste water samples. The first step was the extraction of the PAH from the water into methylene chloride. The efficiency of this step was tested by injecting known amounts of various PAH substances into an aqueous sample and then quantifying the extracted amount. The following table reports the results of several trials for four different compounds. Note the different added amounts in each case. The mean recovered values and the standard deviation of those means are shown at the bottom. Chrysene Benzofluoranthenes Benzo(a)pyrene Dibenz(a,h)anthracene Amount added 1.0 μg/m 3 1.0 μg/m 3 10 μg/m 3 50 μg/m 3 0.64 0.48 6.9 32 0.7 0.96 5.7 22 0.45 0.6 7.8 37 0.72 0.69 5.2 39 0.84 0.08 8 40 0.75 0.2 5.4 30 0.61 0.3 4.7 17 0.92 0.85 5 44 0.8 0.9 6.5 22 0.59 0.69 4 24 0.63 0.88 7.3 27 0.86 0.76 7.6 34 0.58 0.33 5.2 37 0.77 0.39 8.5 46 Mean 0.704 0.579 6.271 32.214 St. Dev 0.129 0.284 1.419 8.877 (a) Calculate the relative standard deviation for each substance. (b) Which substance is most completely recovered? (c) Which substance is most un reliably recovered? (d) What are the 95% confidence limits for the recovery of benzo(a)pyrene?
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2. (6 points) An experiment provided a rapidly and chaotically time varying signal. On average, however, the input signal current was 1.5 μA. It was decided that the signal could be smoothed by integrating it for a fixed length of time. The op amps are driven by a ± 10.0 V source. Here is a basic schematic of the arrangement. (a) What will be the average voltage at point A ? (b) What will be the maximum useful integration time for the circuit? 3. (4 points) A spectrophotometric assay was undertaken with a substance with a molar absorptivity of 4000 M -1 cm -1 . A stock solution is available whose concentration is 0.0195 M. The instrumental path length is 1.000 cm. A final absorbance in the range between 0.4 – 0.5 is desired to calibrate in the range of the unknown solutions. You are to make up 500.0 mL of the calibration solution in a volumetric flask. Do you need to pipette 1.00, 2.00, 3.00, 4.00 or 5.00 mL of the stock solution into the 500.0 mL volumetric flask to provide a solution in this absorbance range? Show your work, 4. (6 points) Look at the six spectra on the next page. The background has already been corrected for all spectra. Read the scales and the titles very carefully. The unknown sample contains a mixture of Cu and Zn ions. What is the concentration of each ion in the unknown?
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This note was uploaded on 05/15/2010 for the course CHEM 3440 taught by Professor Danthomas during the Fall '06 term at University of Guelph.

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midterm 2005 - Chemical Instrumentation CHEM*3440 Mid-Term...

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