Unit 61: Spectroscopy & the Beer-Lambert Law
Practice Problems
Note: problem difficulty is ranked using a star system.
(*) One-star problems are fundamental to the unit, and can be done relatively quickly. Use these problems to introduce yourself to the material.
(**) Two-star problems are more difficult, and require an understanding of one or two key concepts. Use these problems to test your understanding of the material.
(***) Three-star problems are the most difficult, and require some creative thinking in addition to a deep familiarity with multiple key concepts. Use these problems to challenge yourself; if you can complete one of these, you’re on your way to mastering the material.
*Q61.1) Describe the four basic components of an absorption spectrophotometer, and show how they’re arranged with a block diagram
*Q61.2) Describe the four basic components of a fluorescence spectrophotometer, and show how they’re arranged with a block diagram
**Q61.3) Explain the difference between fluorescence and phosphorescence.
**Q61.4) You shine some light of a particular wavelength through a sample and find that 40% of the light was absorbed by the sample.
a) What’s the sample’s absorbance at this wavelength?
b) What’s the sample’s transmittance at this wavelength?
c) What’s the sample’s percent transmittance at this wavelength?
**Q61.5) You perform an absorbance measurement on a sample in a 1cm cuvette at a particular wavelength, and measure a percent transmittance of 20%. What would be the sample’s percent transmittance at the same wavelength if a 5cm cuvette was used?
**Q61.6) At a depth of 1m below the surface of the ocean, 5% of the Sun’s red light is absorbed. How deep below the surface of the ocean would we expect 99% of red light to be absorbed?
***Q61.7) Consider two solutions A and B with the following properties:
Solution A: ε = 1000 L mol⁻¹ cm⁻¹ , C = 5.0 mmol/L
Solution B: ε = 2000 L mol⁻¹ cm⁻¹ , C = 1.0 mmol/L
a) What total absorbance would we measure if the solutions were placed in seperate 1cm cuvettes and placed one behind the other?
b) What total absorbance would we measure if equal volumes of solutions A and B were mixed together, and the resulting solution added to a single 1cm cuvette?