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Absorption Spectroscopy

Can observe color using absorption spectroscopy (which looks at the intensity of light absorbed versus intensity of light transmitted). Absorption Spectroscopy. Shine light (h n ) on complex Molecule absorbs the energy and an e - gets excited from GS orbital to a higher energy orbital

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Absorption Spectroscopy

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  1. Can observe color using absorption spectroscopy (which looks at the intensity of light absorbed versus intensity of light transmitted)

  2. Absorption Spectroscopy • Shine light (hn) on complex • Molecule absorbs the energy and an e- gets excited from GS orbital to a higher energy orbital • e- falls back to GS releasing energy in the form of color

  3. What color would you expect this sample to be? • What would you observe/see with your eyes?

  4. Absorption Spectroscopy • Amount of light absorbed (A) is proportional to concentration sample (c) in molarity: A α c • Beer-Lambert Law: A = εcl ε = molar extinction coefficient (M-1cm-1) l = path length (usually 1 cm)

  5. A student is asked to determine the concentration of a cobalt (II) chloride based on absorption of light. The student is provided with a 0.10 M solution of cobalt (II) chloride with which to prepare standard solutions with concentrations of 0.020 M, 0.040 M, 0.060 M, and 0.080 M. What process is used to turn the 0.10 M solution into the 4 standard solutions? How would you prepare 10.0 mL of the 0.020 M solution?

  6. A student finds the absorbance of the 0.10 M solution at different wavelengths. She creates the graph below. What is the optimum wavelength for analysis?

  7. The student then finds the A of each solution at the optimum and generates the graph below. Does the graph make sense? If the student measured the abs of an unknown solution as 0.275, could she find the [M] of the unknown solution? How?

  8. Concentrations of colored substances are commonly measured by means of a spectrophotometer. Which of the following would ensure correct values for the measured absorbance? • There must be enough sample in the tube to cover the entire path length. • The instrument must be periodically reset using a standard. • The solution must be saturated.

  9. One can use a visible light spectrophotometer for which of the following? • To determine the concentration of a solution of Cu(NO3)2. • To measure the conductivity of a solution of KNO3. • To determine which ions are present in a solution that may contain Na+, Mg2+, Al3+. • To determine the binding energy of electrons. • To determine isotopes in the sample.

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