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Ultraviolet–visible spectroscopy

Chemistry • Analytical Techniques • Knowledge-Based • 5 questions

Ultraviolet–visible spectroscopy Knowledge-Based Download PDF

Chemistry · Applied Chemistry · Chemistry · Analytical Techniques

5 Chemistry practice questions on Ultraviolet–visible spectroscopy. Questions that assess core concepts and understanding.

Questions

  1. 1.

    In routine ultraviolet–visible (UV–Vis) spectroscopy of unsaturated organic compounds, which electronic transition is symmetry-allowed and characteristically yields intense absorption bands with molar absorptivity values exceeding 10,000 L·mol⁻¹·cm⁻¹?

    • A) n → π* transition
    • B) π → π* transition
    • C) n → σ* transition
    • D) σ → σ* transition
  2. 2.

    Which combination of radiation sources is conventionally employed in a UV-Vis spectrophotometer to provide continuous emission across both the ultraviolet and visible wavelength ranges?

    • A) A deuterium discharge lamp for the ultraviolet region and a tungsten-halogen lamp for the visible region
    • B) A hollow-cathode lamp for the ultraviolet region and a mercury-vapor lamp for the visible region
    • C) A tungsten-halogen lamp for the ultraviolet region and a deuterium discharge lamp for the visible region
    • D) A carbon arc lamp for the ultraviolet region and a sodium-vapor lamp for the visible region
  3. 3.

    According to the Beer–Lambert law, what primary phenomenon causes the absorbance versus concentration calibration plot to deviate from linearity at high analyte concentrations (typically above 0.01 M)?

    • A) Electrostatic interactions between neighboring analyte molecules alter their electronic charge distribution and effective molar absorptivity.
    • B) Incident photons undergo destructive interference within the cuvette due to increased solvent refraction.
    • C) The physical path length of the sample cuvette expands under the osmotic pressure generated by concentrated solutes.
    • D) The analyte molecules undergo spontaneous and irreversible photochemical decomposition into non-absorbing radical species.
  4. 4.

    Why are fused silica (quartz) cuvettes required rather than standard optical glass or plastic cuvettes when conducting spectrophotometric measurements in the ultraviolet region below 320 nm?

    • A) Standard glass and plastic cuvettes catalytically degrade photoactive analytes upon exposure to ultraviolet wavelengths.
    • B) Standard glass exhibits an excessively high refractive index that totally reflects ultraviolet light away from the sample.
    • C) Standard plastic cuvettes undergo photochemical thermal expansion that alters the optical path length during analysis.
    • D) Standard glass and plastics strongly absorb ultraviolet radiation below 320 nm, whereas quartz remains optically transparent.
  5. 5.

    In ultraviolet–visible (UV-Vis) spectroscopy, how does an increase in the degree of conjugation within an organic molecule affect its electronic absorption characteristics?

    • A) It causes a hypsochromic shift to shorter wavelengths because the HOMO-LUMO energy gap widens.
    • B) It causes a hypochromic effect without shifting the wavelength because delocalization decreases transition probability.
    • C) It causes a bathochromic shift to longer wavelengths because the energy gap between the HOMO and LUMO decreases.
    • D) It leaves the absorption wavelength unchanged because conjugation only influences vibrational ground states.