Anal Chem 1Inclass Spectrophotometry ProblemsFall 2002

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Anal Chem 1 In-class Spectrophotometry
Problems Fall 2002
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• 1 -
• Calculate the frequency (Hz), wavenumber (cm-1)
  and energy (kJ/photon and kJ/mol photons) of
• a) ultraviolet light with a wavelength of 250 nm
  and
• b) infrared light with a wavelength of 2.50 ?m

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2 - Convert the following transmittance (T)
values to absorbances (A)
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• 3
• The absorbance of a 2.31 x 10-5 M solution is
  0.822 at a wavelength of 266 nm in a 1.00
  cm-cell. Calculate the molar absorptivity at 266
  nm.

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• 4 -
• A 15.0-mg sample of a compound with a formula
  mass of 384.63 was dissolved in a 5-mL volumetric
  flask. A 1.00 mL aliquot was withdrawn, placed
  in a 10-mL volumetric flask and diluted to the
  mark.
• (a) Find the concentration of the sample in the
  5-mL flask.
• (b) Find the concentration of the sample in the
  10-mL flask.
• (c) the 10-mL sample was placed in a 0.500-cm
  cuvet and gave an absorbance of 0.634 at 495nm.
  Find the molar absorptivity at 495nm

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• 5 -
• A compound with a formula mass of 292.16 was
  dissolved in a 5-mL volumetric flask. A 1.00-mL
  aliquot was withdran, placed in a 10-mL
  volumetric flask, and diluted to the mark. The
  absorbance measured at 340 nm was 0.427 in a
  1.000-cm cuvet. The molar absorptivity for this
  compound at 340nm is
• ? 6130 M -1 cm -1.
• (a) Calculate the concentration of compound in
  the cuvet.
• (b) What was the concentration of compound in
  the 5-mL flask?
• (c) How many milligrams of compound were used
  to make the 5-mL solution?

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6 -Two blue solutions, each known to contain
only one absorbing species, had the following
absorbances in a 1.00-cm cuvet.
Do the two solutions contain the same substance?
Explain.
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• 7 -
• A 10.00-mL sample containing benzene was diluted
  to 250.0 mL with hexane and the resulting
  solution had an absorbance of 0.364 at 204 nm in
  a 5.00-cm cuvet. ? for benzene at 204 nm has a
  value of 7900. Calculate the concentration of
  benzene (mg/mL) in the original solution.

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• 8 -
• A 200-mL sample of well water was treated with
  excess hydroxylamine hydrochloride to reduce all
  iron to Fe2. After the addition of acetic
  acid/sodium acetate buffer to adjust the pH, an
  excess of 4,7-diphenyl-1,10-phenanthroline was
  added and the mixture diluted with water to 250
  mL in a volumetric flask. This solution had a
  transmittance of 0.631 at the ?max of 533nm. A
  second solution, prepared by dissolving 72.5 mg
  of pure iron wire in acid was treated similarly
  except the final dilution was to 1.00 liter. A
  10.00-mL aliquot was diluted in 100.0mL
  volumetric flask had a transmittance of 0.515
  using the same cell as used in the first
  measurement. Calculate the ppm of iron in the
  original sample of well water.

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• 9 -
• Soluble phosphate is often analyzed by reacting
  is with a solution of sodium molybdate (Na2MoO4)
  and hydrazine sulfate. An intense blue color
  forms which is the basis of the
  spectrophotometric anlaysis for phosphate. ?max
  at 830nm with ? 24200 M-1 cm-1. Because
  samples of phosphate that are of interest often
  have high concentrations of other species (matrix
  effect) the method of standard addition is
  generally used.
• 100-mL sample of natural water was treated as
  described above with the molybdate and hydrazine
  reagents and diluted in a 250-mL volumetric
  flask. Its absorbance was measured at 830 nm
  using a 1.00-cm cuvet and found to be 0.290.
• A second portion of 100-mL of the sample was
  treated identical to above except 10.00mL of a
  standard phosphate solution whose concentration
  is 1.0 x 10-4 M PO4-3 is added before the two
  reagents and dilution to 250 mL. Its absorbance
  at 830 nm was found to be 0.585 using the same
  1.00-cm cuvet.
• What is the original concentration of phosphate
  in the natural water sample?