Molar Absorptivity

Definition

Molar absorptivity, also known as molar extinction coefficient, is a measure of the ability of a molecule to absorb light at a specific wavelength. It is a fundamental parameter in ultraviolet (UV) spectroscopy, which is used to quantify the concentration of a substance in a sample by measuring the amount of light it absorbs.

5 Must Know Facts For Your Next Test

1. Molar absorptivity is a constant that is specific to a particular molecule and a particular wavelength of light.
2. The higher the molar absorptivity of a molecule, the more strongly it will absorb light at that wavelength.
3. Molar absorptivity is used to determine the concentration of a substance in a sample using the Beer-Lambert law, which states that absorbance is directly proportional to concentration and path length.
4. Conjugation in a molecule can affect its molar absorptivity by altering the energy levels of the electrons and the wavelengths of light that the molecule can absorb.
5. Molar absorptivity is an important parameter in quantitative analysis using UV spectroscopy, as it allows for the accurate determination of the concentration of a substance in a sample.

Review Questions

• Explain how molar absorptivity is related to the Beer-Lambert law and its application in quantitative analysis.
  • Molar absorptivity is a key parameter in the Beer-Lambert law, which states that the absorbance of a substance is directly proportional to its concentration and the path length of the light through the sample. By measuring the absorbance of a sample at a specific wavelength and knowing the molar absorptivity of the substance at that wavelength, the concentration of the substance can be determined. This makes molar absorptivity an essential tool for quantitative analysis using UV spectroscopy, as it allows for the accurate determination of the concentration of a substance in a sample.
• Describe how conjugation in a molecule can affect its molar absorptivity and the wavelength of maximum absorption.
  • Conjugation, the arrangement of alternating single and double bonds in a molecule, can significantly impact the molar absorptivity and the wavelength of maximum absorption. Conjugation lowers the energy gap between the ground state and the excited state of the molecule, allowing it to absorb light at longer wavelengths. This shift in the absorption spectrum is known as a bathochromic shift. Additionally, the degree of conjugation affects the molar absorptivity, with more extensive conjugation generally leading to higher molar absorptivity values. Understanding the relationship between conjugation and molar absorptivity is crucial for interpreting UV spectra and understanding the structural features of organic compounds.
• Analyze the importance of molar absorptivity in the context of 14.7 Ultraviolet Spectroscopy and 14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation.
  • Molar absorptivity is a fundamental concept in ultraviolet (UV) spectroscopy, as described in sections 14.7 and 14.8 of the textbook. In UV spectroscopy, the absorbance of a sample is measured, and this absorbance is directly proportional to the concentration of the absorbing species and its molar absorptivity, as described by the Beer-Lambert law. The molar absorptivity of a molecule is influenced by its structure, particularly the degree of conjugation, as discussed in section 14.8. Molecules with more extensive conjugation tend to have higher molar absorptivity values and exhibit bathochromic shifts in their absorption spectra. Understanding the relationship between molecular structure, conjugation, and molar absorptivity is crucial for interpreting UV spectra and using this analytical technique to identify and quantify organic compounds.

Related terms

Absorbance: The measure of the amount of light that a substance absorbs at a given wavelength, which is directly proportional to the concentration of the substance and the molar absorptivity.

Beer-Lambert Law: The principle that relates the amount of light absorbed by a substance to its concentration and the path length of the light through the sample.

Conjugation: The arrangement of alternating single and double bonds in a molecule, which can affect the molar absorptivity and the wavelength of maximum absorption.