Instructions. This exam is worth 100 points. The points allocated to each section are indicated at the beginning of the section. Some sections allow a choice as to which questions are answered. For these sections, do not answer more parts than requested. No extra credit will be given if more parts than requested are answered. In calculations, be sure to report your answer with the correct number of significant figures. Physical Constants velocity of light in a vacuum: 3.00x108 m/sec Planck's constant: 6.626x10-34 joule-sec Conversion Factors 1 nm = 10-9 m
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Chemistry 325
Instrumental Analysis
February 3, 1995
1. (40 points) Circle the correct answer for each of the following: a. Photomultiplier tubes are not useful as infrared detectors because (1) Infrared photons do not carry enough energy to eject photoelectrons from the photocathode. (2) Infrared sources are too intense and cause degradation of the photocathode. (3) Infrared sources output a light beam that is too large in diameter to be compatible with the photomultiplier tube. (4) None of the above. b. In an experiment based on Beer's Law, (1) Increasing the sample pathlength will increase the transmittance. (2) Increasing absorptivity will increase the transmittance. (3) Decreasing concentration will decrease the transmittance. (4) Increasing the sample pathlength will increase the absorbance. c. A photon whose wavelength is 510 nm is (1) a visible photon (2) an ultraviolet photon (3) an infrared photon (4) a microwave photon d. Which of the following is false? (1) Visible photons carry more energy than infrared photons. (2) Microwave photons carry more energy than infrared photons. (3) Ultraviolet photons carry more energy than visible photons. (4) X-ray photons carry more energy than microwave photons. e. The calculation (0.203 + 0.139) / 2.766 should be reported as (1) 0.1236 (2) 0.123 (3) 0.124 (4) 0.12 f. Given the grating equation, it can be seen that (1) the distance between the grooves of a diffraction grating has no effect on its performance. (2) the angle of incidence of photons striking a diffraction grating must be held constant if the grating is to function properly. (3) the angle at which a photon reflects from the grating is unique for each wavelength. (4) None of the above.
Chemistry 325
Instrumental Analysis
February 3, 1995
g. A D2 arc lamp (1) is the most widely used source of visible photons. (2) is the most widely used source of ultraviolet photons. (3) is the most widely used infrared source. (4) has a continuous output spanning the ultraviolet and visible ranges. h. Double beam spectrometers (1) employ an optical chopper or beam director to alternate the source beam between the reference and sample compartments. (2) usually employ a motor drive to move the dispersive element in the monochromator. (3) automatically correct for changes in source output with wavelength. (4) All of the above. i. Which of the following is true? (1) If an analysis is accurate, it must also be precise. (2) A precise analytical measurement will always have a small relative standard deviation. (3) If an analysis is reproducible, it will be accurate. (4) None of the above. j. The presence of I2 in a tungsten halogen source (1) causes the source output to shift to longer wavelengths. (2) causes the source output to shift to shorter wavelengths. (3) increases the lifetime of the filament by preventing oxygen from reacting with the tungsten filament. (4) increases the lifetime of the bulb by causing vaporized tungsten to redeposit on the filament. 2. (40 points) Answer five of the following in one or two sentences: a. When a molecule absorbs light, it increases in energy. It then returns to a lower energy state by releasing energy. Name two forms of energy that the molecule can release.
b. Name one advantage offered by the method of standard additions over a conventional analysis based on the construction of a calibration curve.
Chemistry 325
Instrumental Analysis
February 3, 1995
c. What is the function of a monochromator in a spectrometer that employs a tungsten halogen source?
d. What is the purpose of the chain of dynodes in a photomultiplier tube?
e. In a ultraviolet-visible spectrometer, why is the sample placed after the monochromator?
f. Compute the energy in joules of a photon whose wavelength is 321 nm.
3. (10 points) You are given the following information regarding the precision of four pipets: volume 1 mL 7 mL 9 mL 15 mL
standard deviation 0.006 mL 0.02 mL 0.02 mL 0.03 mL
If you wish to deliver 16 mL, compute the error in each of the two possible delivery methods: (1) 15 mL + 1 mL or (2) 9 mL + 7 mL.
Chemistry 325
Instrumental Analysis
February 3, 1995
4. (10 points) The drug tolbutamine (FW=270) has a molar absorptivity of 703 cm-1mole-1L at 262 nm. One tablet weighing 1.113 g is dissolved in water and diluted to a volume of 2000 mL in a volumetric flask. If the solution exhibits an absorbance at 262 nm equal to 0.687 (1.0 cm pathlength cell), what is the weight percent of tolbutamine in the tablet? .687 = (703)(1) C = 0.000977 M x 2 L = 0.00195 moles 0.00195 moles x 270 g/mole = 0.528 g 0.528 g / 1.113 g = 0.474 x 100 = 47.4%
