UV‑Visible spectroscopy: Beer–Lambert law, electronic transitions, chromophores and auxochromes - One Line Questions

1. Which functional group is a classic example of a chromophore that absorbs in the UV region? — -C=O (carbonyl)
2. What is the absorbance of a 0.005 M solution of a compound with a molar absorptivity of 10,000 M⁻¹cm⁻¹ in a 1 cm cuvette? — 0.5
3. If a solution has an absorbance of 0.500 and the path length is 1 cm, what is the molar absorptivity if the concentration is 0.01 M? — 50 M⁻¹cm⁻¹
4. In the Beer–Lambert law, A = εbc, what does 'ε' represent? — Molar absorptivity (or extinction coefficient)
5. In the Beer–Lambert law, A = εbc, what does 'b' represent? — Path length (or cuvette width)
6. The energy difference between the ground state and the excited state of a molecule is inversely proportional to: — Wavelength of absorbed light
7. What is the effect of increasing the path length (b) in the Beer-Lambert law, assuming concentration and molar absorptivity are constant? — Absorbance increases proportionally.
8. A hypsochromic shift (or blue shift) in UV-Visible spectroscopy indicates: — A shift to shorter wavelength (higher energy).
9. The phenomenon where the absorption intensity decreases due to structural changes or interactions is called: — Hypochromism
10. The phenomenon where the absorption intensity increases is called: — Hyperchromism
11. Which of the following is an example of an auxochrome? — Hydroxyl group (-OH)
12. Which of the following is NOT a typical chromophore? — Saturated C-C single bond
13. In UV-Vis spectroscopy, a shift to longer wavelength is called a bathochromic shift, and a shift to shorter wavelength is called a hypsochromic shift. What causes these shifts? — Changes in molecular structure, such as conjugation or the presence of auxochromes
14. A molecule that absorbs UV-Visible light is called a: — Chromophore
15. When a molecule absorbs light in the UV-Visible region, what happens to the electrons? — Electrons are promoted from a lower energy orbital to a higher energy orbital.
16. Which of the following functional groups would be expected to exhibit a significant absorption in the UV region due to π → π* transitions? — Benzene (C6H6)
17. The Beer–Lambert law is most accurate under what conditions? — Low concentrations and monochromatic light
18. The Beer–Lambert law is applicable to: — Homogeneous solutions
19. The shift of absorption maximum to longer wavelength (lower energy) due to the presence of an auxochrome is known as: —
20. Which of the following statements regarding UV-Vis spectroscopy is incorrect? — It is highly effective for analyzing saturated hydrocarbons.
21. What effect does the addition of an auxochrome typically have on the intensity of absorption in UV-Visible spectroscopy? — It usually increases the intensity (hyperchromic effect).
22. A sample exhibits maximum absorbance at 550 nm. If the concentration is doubled, what will be the new maximum absorbance, assuming the Beer-Lambert law holds? — It will remain 550 nm.
23. The term 'chromophore' is derived from Greek words meaning: — Color and carrier
24. Conjugated systems, like polyenes, exhibit absorption at longer wavelengths compared to isolated double bonds primarily due to: — Lower energy gap between HOMO and LUMO.
25. Which of the following electronic transitions is typically observed in the UV-Visible region of the electromagnetic spectrum for simple alkanes? — σ → σ*
26. The transition responsible for the absorption of light by saturated hydrocarbons like methane is: — σ → σ*
27. Which transition typically occurs at the shortest wavelengths (highest energy) among the common ones? — σ → σ*
28. An auxochrome often contains: — Lone pairs of electrons or polar groups
29. The Beer–Lambert law is a linear relationship between absorbance and: — Both concentration and path length
30. Which factor is LEAST likely to cause a deviation from the Beer–Lambert law? — Very low concentration
31. A molecule with extended conjugation will typically show absorption at: — Longer wavelengths (lower energy)
32. What is the fundamental principle described by the Beer–Lambert law in UV-Visible spectroscopy? — The absorbance of a solution is directly proportional to the concentration of the analyte and the path length of the light beam.
33. The molar absorptivity (ε) is a measure of: — How strongly a chemical species absorbs light at a particular wavelength.
34. The sensitivity of a UV-Visible spectrophotometric method is directly related to: — The molar absorptivity (ε)
35. In the context of UV-Visible spectroscopy, what does 'λmax' signify? — The wavelength where the absorbance is maximum for a given transition.
36. Which of the following statements about auxochromes is FALSE? — They are the primary light-absorbing parts of a molecule.
37. Why are n → π* transitions often weaker (lower molar absorptivity) than π → π* transitions? — They are symmetry-forbidden or involve poor orbital overlap.
38. What is the primary role of an auxochrome in UV-Visible spectroscopy? — To modify the absorption characteristics (wavelength and intensity) of a chromophore.
39. What is the primary reason for using a reference blank in UV-Visible spectroscopy? — To measure the absorbance of the solvent and cuvette, allowing it to be subtracted from the sample's absorbance.
40. Deviations from the Beer–Lambert law can occur due to: — Chemical interactions between analyte molecules (e.g., association, dissociation)
41. Which solvent property can influence the position and intensity of UV-Visible absorption bands? — Polarity
42. The longest wavelength absorption band in a molecule is generally referred to as the: — λmax (wavelength of maximum absorbance)
43. Which type of electronic transition requires the least amount of energy and is thus observed at the longest wavelengths in UV-Visible spectroscopy? — n → π*
44. Which electronic transition involves excitation of a non-bonding electron to an antibonding π orbital? — n → π*
45. Which electronic transition involves excitation of a bonding π electron to an antibonding π* orbital? — π → π*
46. In transition metal complexes, which type of electronic transition often leads to colored compounds? — d → d transitions
47. Which electronic transition is most likely to be observed in a molecule like acetone (CH3COCH3)? — n → π*
48. A compound absorbs strongly at 250 nm. This absorption is likely due to which type of transition? — π → π* in a conjugated system
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