Chem. 312, Spring 2007, Practice Exam # 2. Due at class time Tuesday April 17th
Name(s): ……………………
If you do this homework as a collaborative exercise all members of the collaboration should sign here (or the front page of the solutions you submit) to indicate that it was a genuine collaboration.
Read everything before doing anything. Answer all the questions in any order that seems logical. Not all the questions are weighted equally; plan your time accordingly. Merit is attached to clarity of presentation (up to 5 points will be deducted). There are six questions in total. If you hand in solutions to the practice exam by the due date you will receive up to 10% towards the real exam.
1. You are the manager of an analytical laboratory that has been awarded a contract to determine cadmium in the environment in Western Massachusetts. Your lab already has molecular spectrometry instrumentation, but you are thinking about maybe adding electrochemical and atomic spectroscopy instruments to help with the variety of analytical problems that are likely to arrive. The first task is the determination of cadmium in the Quabbin reservoir, for which you need a procedure with a detection limit of a few ppb. Do you buy (a) an ion-sensitive electrode and high impedance voltmeter, (b) an instrument for anodic stripping voltammetry or (c) an atomic absorption spectrometer with a graphite furnace atomizer? (5 points) Explain your decision (5 points).
2. The sales representative for the company that sells the ion-sensitive electrode has brought a system to demonstrate. She has two standard solutions: one containing 100 ppb and the other 10 ppm. She records the voltmeter responses for these solutions as 20.0 mV and 50.0 mV. You give her an “unknown” solution for which she measures the response to be 40.0 mV. What concentration does she calculate for your test solution in ng mL-1? (15 points)
3. You decide to evaluate a flame atomic absorption procedure for the determination of Cd in a contaminated soil extract. To 50.0 mL of a standard solution containing 500 ppb Cd is added 10 mL of a 0.5% solution of APDC (ammonium pyrrolidine dithiocarbamate), and the solution is shaken for 2 min. Then 5.0 mL of methyl isobutyl ketone (MIBK, which is immiscible with water) is added and the mixture shaken for a further 2 min. The layers are allowed to separate and the the MIBK layer is sprayed into the spectrometer and the absorbance measured. Fifty mL of water are taken through the same procedure, as is 50.0 mL of a soil extract solution. If the absorbances of the standard, blank and sample solutions are 0.240, 0.040 and 0.120, respectively, what is the concentration of Cd in the soil extract solution in ppb? (20 points) Assume that Cd forms an MIBK-extractable compound with APDC. If the mass of soil taken was 250 mg, calculate the concentration of the extractable Cd in ppm (on a mass/mass basis). (5 points) Assume all the cadmium has been extracted.
4. The stock cadmium solution from which the atomic absorption standard was made is labeled “1000 ppm”. As you don’t know how many of these digits are significant, you decide to analyze the solution by spectrophotometric titration with EDTA in the presence of Eriochrome Black T indicator. You select a wavelength at which the Cd-indicator complex absorbs radiation, but the free indicator does not. Sketch the shape of the titration plot (i.e. the plot of absorbance as a function of volume of titrant added. (5 points). The end point for the titration of 25.00 mL of the cadmium solution with 1.000 x 10-2 M EDTA solution is 22.04 mL. What is the concentration of the stock cadmium solution? (15 points). Does this result change your answers to question 3 above? (5 points)
5. Cadmium reacts with 4-(2pyridylazo)-resorcinol (PAR) to form a colored complex with molar absorptivity 5,000 L mol-1 cm-1 at 500 nm. If a test solution absorbs 60% of the radiation in a 1.00-cm cell, what is the concentration of cadmium in the solution, in ppm, if the Cd-PAR complex contains only one cadmium atom? (15 points) Estimate the limit of detection (in ppb) for the spectrophotometric determination of Cd with this reagent. (5 points). The atomic weight of cadmium is 112.411 g mol-1.
6. Write instructions for the dilution of the stock “1000” ppm solution to give the “500” ppb solution used in the procedure described in question 3 above. (10 points)
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