UV‑Visible spectroscopy: Beer–Lambert law, electronic transitions, chromophores and auxochromes - Question Bank

1. 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?
A) Changes in temperature
B) Changes in pressure
C) Changes in molecular structure, such as conjugation or the presence of auxochromes
D) Changes in the path length of the cuvette
2. The Beer–Lambert law is applicable to:
A) Homogeneous solutions
B) Heterogeneous mixtures
C) Solid samples only
D) Gaseous samples only
3. Which of the following functional groups would be expected to exhibit a significant absorption in the UV region due to π → π* transitions?
A) Ethanol (CH3CH2OH)
B) Ethane (CH3CH3)
C) Benzene (C6H6)
D) Diethyl ether (CH3CH2OCH2CH3)
4. The energy difference between the ground state and the excited state of a molecule is inversely proportional to:
A) Absorbance
B) Concentration
C) Wavelength of absorbed light
D) Molar absorptivity
5. A compound absorbs strongly at 250 nm. This absorption is likely due to which type of transition?
A) σ → σ*
B) n → σ*
C) π → π* in a conjugated system
D) n → π* in a carbonyl group
6. What is the effect of increasing the path length (b) in the Beer-Lambert law, assuming concentration and molar absorptivity are constant?
A) Absorbance remains constant.
B) Absorbance decreases.
C) Absorbance increases proportionally.
D) Absorbance becomes zero.
7. Which of the following statements regarding UV-Vis spectroscopy is incorrect?
A) It is primarily used for quantitative analysis.
B) It relies on electronic transitions of molecules.
C) It is highly effective for analyzing saturated hydrocarbons.
D) The Beer-Lambert law is a fundamental equation used.
8. In the context of UV-Visible spectroscopy, what does 'λmax' signify?
A) The wavelength of minimum absorbance.
B) The wavelength at which the molar absorptivity is zero.
C) The wavelength where the absorbance is maximum for a given transition.
D) The wavelength range over which the instrument operates.
9. The sensitivity of a UV-Visible spectrophotometric method is directly related to:
A) The path length (b)
B) The molar absorptivity (ε)
C) The concentration (c)
D) The wavelength of maximum absorbance (λmax)
10. Which electronic transition is most likely to be observed in a molecule like acetone (CH3COCH3)?
A) σ → σ*
B) π → π*
C) n → σ*
D) n → π*
11. 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?
A) It will remain 550 nm.
B) It will decrease.
C) It will double.
D) It will become immeasurable.
12. When a molecule absorbs light in the UV-Visible region, what happens to the electrons?
A) Electrons are completely removed from the molecule.
B) Electrons are promoted from a lower energy orbital to a higher energy orbital.
C) Electrons vibrate more intensely.
D) Electrons are transferred to the solvent.
13. Which of the following statements about auxochromes is FALSE?
A) They are typically electron-donating groups.
B) They can shift the absorption maximum to longer wavelengths.
C) They are the primary light-absorbing parts of a molecule.
D) They often contain lone pairs of electrons.
14. The term 'chromophore' is derived from Greek words meaning:
A) Light and carrier
B) Color and carrier
C) Energy and absorber
D) Wavelength and changer
15. Which factor is LEAST likely to cause a deviation from the Beer–Lambert law?
A) Polychromatic radiation
B) Concentration-dependent chemical equilibria
C) Scattering of light
D) Very low concentration
16. What is the primary reason for using a reference blank in UV-Visible spectroscopy?
A) To increase the absorbance of the analyte.
B) To measure the absorbance of the solvent and cuvette, allowing it to be subtracted from the sample's absorbance.
C) To ensure the Beer-Lambert law is always obeyed.
D) To provide a calibration standard for concentration determination.
17. Why are n → π* transitions often weaker (lower molar absorptivity) than π → π* transitions?
A) They involve more electrons.
B) They are symmetry-forbidden or involve poor orbital overlap.
C) They occur at much shorter wavelengths.
D) They require very high concentrations.
18. The Beer–Lambert law is a linear relationship between absorbance and:
A) Path length only
B) Concentration only
C) Molar absorptivity only
D) Both concentration and path length
19. A molecule with extended conjugation will typically show absorption at:
A) Shorter wavelengths (higher energy)
B) Longer wavelengths (lower energy)
C) The same wavelength as isolated chromophores
D) Only in the infrared region
20. The longest wavelength absorption band in a molecule is generally referred to as the:
A) π → π* transition
B) σ → σ* transition
C) n → π* transition
D) λmax (wavelength of maximum absorbance)
21. Which solvent property can influence the position and intensity of UV-Visible absorption bands?
A) Viscosity
B) Boiling point
C) Polarity
D) Density
22. In transition metal complexes, which type of electronic transition often leads to colored compounds?
A) σ → σ*
B) π → π*
C) d → d transitions
D) n → π*
23. The phenomenon where the absorption intensity increases is called:
A) Bathochromic shift
B) Hypsochromic shift
C) Hyperchromism
D) Hypochromism
24. The phenomenon where the absorption intensity decreases due to structural changes or interactions is called:
A) Bathochromic shift
B) Hypsochromic shift
C) Hyperchromism
D) Hypochromism
25. An auxochrome often contains:
A) Only sigma bonds
B) Lone pairs of electrons or polar groups
C) Non-polar alkyl chains
D) A high degree of unsaturation
26. Which of the following is NOT a typical chromophore?
A) C=C double bond
B) C=O carbonyl group
C) Aromatic ring
D) Saturated C-C single bond
27. 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?
A) 0.0005
B) 0.05
C) 0.5
D) 5.0
28. 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?
A) 5 M⁻¹cm⁻¹
B) 50 M⁻¹cm⁻¹
C) 500 M⁻¹cm⁻¹
D) 0.05 M⁻¹cm⁻¹
29. The molar absorptivity (ε) is a measure of:
A) The concentration of the absorbing species.
B) The path length of the light through the sample.
C) How strongly a chemical species absorbs light at a particular wavelength.
D) The total amount of light absorbed by the sample.
30. Conjugated systems, like polyenes, exhibit absorption at longer wavelengths compared to isolated double bonds primarily due to:
A) Lower energy gap between HOMO and LUMO.
B) Higher energy gap between HOMO and LUMO.
C) Presence of n electrons.
D) Increased molar absorptivity.
31. Which transition typically occurs at the shortest wavelengths (highest energy) among the common ones?
A) n → π*
B) π → π*
C) n → σ*
D) σ → σ*
32. The transition responsible for the absorption of light by saturated hydrocarbons like methane is:
A) n → π*
B) π → π*
C) n → σ*
D) σ → σ*
33. Which electronic transition involves excitation of a bonding π electron to an antibonding π* orbital?
A) σ → σ*
B) n → σ*
C) π → π*
D) n → π*
34. Which electronic transition involves excitation of a non-bonding electron to an antibonding π orbital?
A) σ → σ*
B) π → π*
C) n → σ*
D) n → π*
35. Deviations from the Beer–Lambert law can occur due to:
A) Use of monochromatic light
B) Low analyte concentration
C) Chemical interactions between analyte molecules (e.g., association, dissociation)
D) Using a suitable solvent
36. The Beer–Lambert law is most accurate under what conditions?
A) High concentrations and complex solutions
B) Low concentrations and monochromatic light
C) Broadband light sources and high concentrations
D) Solutions with multiple absorbing species and varying path lengths
37. What effect does the addition of an auxochrome typically have on the intensity of absorption in UV-Visible spectroscopy?
A) It usually decreases the intensity (hypochromic effect).
B) It usually increases the intensity (hyperchromic effect).
C) It has no effect on the intensity.
D) It causes a shift to shorter wavelengths only.
38. A hypsochromic shift (or blue shift) in UV-Visible spectroscopy indicates:
A) An increase in molar absorptivity.
B) A decrease in molar absorptivity.
C) A shift to longer wavelength (lower energy).
D) A shift to shorter wavelength (higher energy).
39. The shift of absorption maximum to longer wavelength (lower energy) due to the presence of an auxochrome is known as:
A) Hypsochromic shift
B) Bathochromic shift
C) Hyperchromic effect
D) Hypochromic effect
40. Which of the following is an example of an auxochrome?
A) Benzene ring
B) Ethylene group
C) Hydroxyl group (-OH)
D) Carbonyl group (-C=O)
41. Which functional group is a classic example of a chromophore that absorbs in the UV region?
A) -OH (hydroxyl)
B) -CH3 (methyl)
C) -C=O (carbonyl)
D) -NH2 (amino)
42. What is the primary role of an auxochrome in UV-Visible spectroscopy?
A) To be the primary absorbing group in a molecule.
B) To modify the absorption characteristics (wavelength and intensity) of a chromophore.
C) To emit light after absorbing UV-Visible radiation.
D) To facilitate electron delocalization within a conjugated system.
43. A molecule that absorbs UV-Visible light is called a:
A) Chromophore
B) Auxochrome
C) Fluorophore
D) Phosphor
44. Which type of electronic transition requires the least amount of energy and is thus observed at the longest wavelengths in UV-Visible spectroscopy?
A) σ → σ*
B) n → σ*
C) π → π*
D) n → π*
45. Which of the following electronic transitions is typically observed in the UV-Visible region of the electromagnetic spectrum for simple alkanes?
A) n → π*
B) π → π*
C) σ → σ*
D) d → d
46. In the Beer–Lambert law, A = εbc, what does 'b' represent?
A) Absorbance
B) Molar absorptivity
C) Concentration
D) Path length (or cuvette width)
47. In the Beer–Lambert law, A = εbc, what does 'ε' represent?
A) Absorbance
B) Molar absorptivity (or extinction coefficient)
C) Concentration
D) Path length
48. What is the fundamental principle described by the Beer–Lambert law in UV-Visible spectroscopy?
A) The absorbance of a solution is directly proportional to the concentration of the analyte and the path length of the light beam.
B) The transmittance of a solution is directly proportional to the concentration of the analyte and the path length of the light beam.
C) The absorbance of a solution is inversely proportional to the concentration of the analyte and the path length of the light beam.
D) The transmittance of a solution is inversely proportional to the concentration of the analyte and the path length of the light beam.
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