midterm 2003

# midterm 2003 - CHEM*3440 Instrumental Methods Mid-Term...

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CHEM*3440 Instrumental Methods Mid-Term Examination October 28, 2003 Duration: Two Hours 1. (5 points) A real battery actually behaves more like a voltage source in series with a resistance, called its internal resistance. (This arises from the fact that the chemical reactions have finite rate constants and can produce electrons only at a finite rate.) An older 1.5 V dry cell battery, when shorted out with a wire of insignificant resistance, is able to deliver only 5.0 mA of current. What is the internal resistance of the battery? 2. (10 points) You are analyzing a 2.0 g biological tissue sample, trying to determine if Ni is present. Your experiment is sensitive down to the 0.1 ppb level. The tissue was obtained by slicing it with a stainless steel scalpel. The stainless steel is 10% Ni. (a) What mass of the scalpel rubbing off would contribute a sufficient quantity of Ni to give a positive (0.1 ppb) test result? (b) Assume that the density of the steel is 7.0 g/cm 3 and that the scalpel lost the material in a single, perfectly cubic chip. What is the length of the edge of that cube? (For comparison, a red blood cell has a diameter of about 10 μm.) V R int + + + 1 k W 1 k W 100 k W 100 k W 10 μF V DC V AC A B C 3. (15 points) Analyze the accompanying op amp circuit. There are two input signals: V DC is a constant 1.00 V and V AC is a sine wave of amplitude ±1V and a frequency of 10 Hz. The op amps are being powered by ±10 V. Carefully graph a voltage-time plot at the three indicated locations, A, B, and C. Show all calculations you used to arrive at any time or voltage relationships. Draw your curves carefully and emphasize the distinguishing features. 4. (5 points) A spectrometric assay is designed to work with a final concentration of nicotine in the range between 0.005 and 0.020 mg/L. If a 500.0 mL sample of urine is collected and all of the nicotine present is concentrated during sample preparation into a final solution volume of 10.00 mL, what is the minimum concentration of nicotine that can be measured by the assay? (Report your results in μg/L).

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2 6. (16 points: 3 x 5 points + 1 x 1 point) We are passing a 1 A DC current through a 10 kW resistor at 20 ˚C. It is amplified by an amplifier with a bandwidth of 20 kHz. (a) What is the rms thermal noise voltage produced? (b) What is the rms shot noise current produced? (c) In this resistor, what is the rms shot noise voltage produced by the shot noise current? (d) Which noise source dominates under these conditions, thermal or shot? + + V input A B 1 2 3 cell 5. (10 points) Here is an extension of our class discussion of the op amp circuit that represents a simplified three-electrode electrochemical cell. Two additional circuit elements have been added (one is NOT the booster amplifier we discussed). Bear in mind that the reference electrode should draw negligible current, the counter electrode is only present to carry the main current, and that the working electrode is held at virtual ground.
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