Transcript multiple choice

Lecture 1
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PRACTICE TEST
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The active agent in common household bleach is Sodium hypochlorite. Calculate the
pH of 0.15M NaOCl solution (Ka HOCl = 3.0 x 10-8 )
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Which of the following is false?
A. Visible photons carry more energy than infrared photons.
B. Microwave photons carry more energy than infrared photons.
C. Ultraviolet photons carry more energy than visible photons.
D. X-ray photons carry more energy than microwave photons.
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A D2 arc lamp
A. is the most widely used source of visible photons.
B. is the most widely used source of ultraviolet photons.
C. is the most widely used infrared source.
D. has a continuous output spanning the ultraviolet and visible ranges.
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Double beam spectrometers
A. employ an optical chopper or beam director to alternate the source beam between
the reference and sample compartments.
B. usually employ a motor drive to move the dispersive element in the monochromator.
C. automatically correct for changes in source output with wavelength.
D. All of the above.
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Which of the following is true?
A. If an analysis is accurate, it must also be precise.
B. A precise analytical measurement will always have a small relative standard
deviation.
C. If an analysis is reproducible, it will be accurate.
D. None of the above.
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Sketch a calibration curve generated using the method of
standard additions. Name one advantage offered by the
method of standard addition over a conventional analysis.
SHORT ANSWERS
In a ultraviolet-visible spectrometer, why is the sample placed
after the monochromator?
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Compute the energy in joules of a photon whose wavelength is 321 nm
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List five of the six numerical criteria for selecting analytical methods.
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How does absorption and emission of radiation by atoms differ from that absorbed
and emitted by molecules. Comment on the absorption spectrum of an atomic vapor
versus a molecular vapor.
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Compare and contrast continuum and lines sources. Give at least one example of
each.
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What is a laser? What are the advantages of using lasers as spectroscoptic sources.
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How many fundamental modes of vibration are predicted for methane (CH4)? Diagram
one vibrational mode that you would expect to not be IR active.
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Sketch a diagram of a single beam UV-VIS spectrophotometer. Label the parts.
Describe the basic difference between atomic emission and atomic absorption
spectroscopy.
Carbon monoxide (CO) can be determined at trace levels using IR absorption
spectrophotometry. Using a 100.0-cm pathlength gas cell, a standard containing 10.0 ppm
CO gave an absorbance of 0.050 at 2170 cm-1. Calculate the concentration (ppm) of a gas
sample.
PROBLEM SOLVING
Molar absorptivity data for the cobalt and nickel complexes with
2,3-quinoxalinedithiol are:
, nm

510 nm
5520
36400
656 nm
17500
1240
(Ni)

(Co)
A 0.425 g sample was dissolved and diluted to 50 mL. A 25.0 mL
aliquot was treated to eliminate interferences; after addition of
2,3-quinaxalinedithiol, the volume was adjusted to 50.0 mL. This
solution has absorbance of 0.446 at 510 nm and 0.326 at 656nm.
Calculate the ppm of Ni and Co in the sample.
Li was determined by atomic emission with the method
of standard addition. Prepare a standard addition graph
to find the concentration of Li. The standard contained
1.62 g Li/mL.
Unknown, mL
Standard, mL
Final Volume, mL
Emission intensity
10.0
0.00
100.0
309
10.0
5.00
100.0
452
10.0
10.00
100.0
600
10.0
15.00
100.0
765
10.0
20.00
100.0
906
SAMPLE PROBLEM