MCQ in UV spectroscopy (Practice test)

Interactive MCQ Quiz: Principle of UV Spectroscopy

Question 1 of 30

1. UV spectroscopy is primarily based on which type of transition?

a) Rotational

b) Vibrational

c) Electronic

d) Nuclear

Answer: c) Electronic

Explanation: UV spectroscopy deals with electronic transitions of molecules as they absorb UV radiation, promoting electrons from lower to higher energy levels.

2. The commonly used wavelength range in UV spectroscopy is:

a) 1–20 nm

b) 200–400 nm

c) 400–700 nm

d) 700–1000 nm

Answer: b) 200–400 nm

Explanation: UV region lies between 200–400 nm, below which is vacuum UV and above is visible light region.

3. Beer-Lambert law relates absorbance with which parameters?

a) Concentration and path length

b) Wavelength and frequency

c) Solvent polarity and temperature

d) Pressure and molecular weight

Answer: a) Concentration and path length

Explanation: Beer-Lambert law states A = εcl, where absorbance is proportional to molar absorptivity (ε), concentration (c), and path length (l).

4. What does ε represent in Beer-Lambert law?

a) Emission constant

b) Molar absorptivity (extinction coefficient)

c) Excitation energy

d) Electron density

Answer: b) Molar absorptivity (extinction coefficient)

Explanation: ε is the molar absorptivity, a measure of how strongly a compound absorbs UV light at a given wavelength.

5. Which type of molecules are mostly studied in UV spectroscopy?

a) Saturated alkanes

b) Compounds with conjugated double bonds

c) Noble gases

d) Salts

Answer: b) Compounds with conjugated double bonds

Explanation: Conjugated double bonds absorb strongly in the UV region due to π → π* transitions, making them ideal for UV analysis.

6. What is the principle of absorption in UV spectroscopy?

a) Excitation of atoms to nuclear levels

b) Transition of electrons between molecular orbitals

c) Vibration of chemical bonds

d) Emission of gamma rays

Answer: b) Transition of electrons between molecular orbitals

Explanation: UV absorption occurs when photon energy promotes electrons from bonding or non-bonding orbitals to antibonding orbitals.

7. Which transition requires the highest energy?

a) n → π*

b) π → π*

c) σ → σ*

d) n → σ*

Answer: c) σ → σ*

Explanation: σ → σ* transitions require the most energy as they involve breaking strong sigma bonds, occurring in the deep UV region.

8. Which transition is most common in carbonyl groups?

a) π → σ*

b) n → π*

c) σ → σ*

d) π → π*

Answer: b) n → π*

Explanation: In carbonyl groups, non-bonding electrons on oxygen undergo n → π* transition, typically appearing around 280-290 nm.

9. Chromophores are defined as:

a) Atoms absorbing infrared radiation

b) Part of molecule responsible for UV absorption

c) Heavy metal ions

d) Aromatic solvent groups

Answer: b) Part of molecule responsible for UV absorption

Explanation: Chromophores are functional groups or molecular moieties that absorb UV-visible radiation, responsible for electronic transitions.

10. Which group is an example of auxochrome?

a) Nitro group (-NO₂)

b) Amino group (-NH₂)

c) Carbonyl group (C=O)

d) Phenyl group

Answer: b) Amino group (-NH₂)

Explanation: Auxochromes like -OH, -NH₂ intensify absorption of chromophores by extending conjugation through resonance effects.

11. UV absorption occurs when electrons are promoted from:

a) Ground to excited nuclear state

b) Lower to higher electronic energy levels

c) Ionic to covalent bonds

d) Solvent to solute molecules

Answer: b) Lower to higher electronic energy levels

Explanation: UV radiation provides energy to excite electrons from occupied molecular orbitals to unoccupied higher energy orbitals.

12. Which factor affects the absorption maxima (λmax) in UV spectra?

a) Solvent polarity

b) Temperature

c) pH of the solution

d) All of the above

Answer: d) All of the above

Explanation: Solvent effects, temperature changes, and pH variations can all cause shifts in λmax by altering the electronic environment.

13. The shift to longer wavelength in UV spectra is called:

a) Hypsochromic shift (blue shift)

b) Bathochromic shift (red shift)

c) Hypochromic effect

d) Hyperchromic effect

Answer: b) Bathochromic shift (red shift)

Explanation: Bathochromic shift is a red shift indicating absorption at longer wavelength, often due to extended conjugation.

14. The decrease in absorption intensity without wavelength change is called:

a) Hyperchromic effect

b) Hypochromic effect

c) Bathochromic shift

d) Hypsochromic shift

Answer: b) Hypochromic effect

Explanation: Hypochromic effect refers to decreased absorption intensity without change in wavelength, often due to aggregation or H-bonding.

15. Which solvent is commonly used in UV spectroscopy due to low UV absorption?

a) Water

b) Hexane

c) Ethanol

d) Cyclohexane

Answer: c) Ethanol

Explanation: Ethanol has a low UV cutoff (~210 nm) and minimal background absorption, making it suitable as a UV solvent.

16. What type of electronic transition is characteristic of aromatic compounds?

a) n → π*

b) π → π*

c) σ → σ*

d) d-d transitions

Answer: b) π → π*

Explanation: Aromatic compounds show strong π → π* transitions due to their conjugated π-electron systems, typically around 250-280 nm.

17. Which component in UV spectrophotometer selects narrow wavelength bands?

a) Monochromator

b) Photomultiplier tube

c) Light source

d) Sample holder

Answer: a) Monochromator

Explanation: The monochromator (usually a diffraction grating) disperses polychromatic light and selects specific wavelengths for analysis.

18. In UV spectrophotometry, absorbance is calculated from:

a) Transmitted light intensity only

b) Incident and transmitted light intensities

c) Reflected light intensity

d) Emitted light intensity

Answer: b) Incident and transmitted light intensities

Explanation: Absorbance A = log(I₀/I), where I₀ is incident intensity and I is transmitted intensity after passing through sample.

19. Which of the following is NOT a chromophore?

a) C=C double bond

b) Carbonyl group (C=O)

c) Saturated alkane chain

d) Benzene ring

Answer: c) Saturated alkane chain

Explanation: Saturated alkanes lack π-electrons and don't absorb UV light significantly; they are transparent in UV region.

20. Which causes a hypsochromic shift (blue shift)?

a) Increase in conjugation

b) Addition of electron-withdrawing groups

c) Increase in solvent polarity for π → π* transitions

d) Addition of auxochromes

Answer: b) Addition of electron-withdrawing groups

Explanation: Electron-withdrawing groups can destabilize excited states more than ground states, causing shift to shorter wavelengths.

21. The term "hyperchromic effect" refers to:

a) Increase in absorption intensity

b) Shift to shorter wavelength

c) Shift to longer wavelength

d) Decrease in absorption intensity

Answer: a) Increase in absorption intensity

Explanation: Hyperchromic effect refers to increased molar absorptivity (ε) or absorption intensity without wavelength shift.

22. What is the role of the blank solution in UV spectrophotometry?

a) To calibrate wavelength scale

b) To set zero absorbance baseline

c) To increase sample absorbance

d) To act as a reference peak

Answer: b) To set zero absorbance baseline

Explanation: The blank (containing only solvent) corrects for solvent absorbance and instrument background, establishing A = 0 baseline.

23. Which factor does NOT affect absorbance in UV spectroscopy according to Beer-Lambert law?

a) Sample concentration

b) Path length of cuvette

c) Molar absorptivity

d) Atmospheric pressure

Answer: d) Atmospheric pressure

Explanation: According to Beer-Lambert law (A = εcl), only molar absorptivity, concentration, and path length affect absorbance.

24. When vibrational and electronic transitions combine in UV spectra, this results in:

a) Hyperchromic effect

b) Vibronic fine structure

c) Complete absence of absorption

d) Fluorescence emission

Answer: b) Vibronic fine structure

Explanation: Vibronic coupling between electronic and vibrational transitions creates fine structure or multiple peaks in UV spectra.

25. What is the UV cutoff wavelength for standard quartz cuvettes?

a) 220 nm

b) 190 nm

c) 330 nm

d) 400 nm

Answer: b) 190 nm

Explanation: Quartz cuvettes transmit UV light effectively down to approximately 190 nm, making them suitable for UV spectroscopy.

26. UV-visible spectroscopy is often combined with which technique for complete structural elucidation?

a) Infrared spectroscopy

b) Mass spectrometry

c) NMR spectroscopy

d) All of the above

Answer: d) All of the above

Explanation: UV-Vis provides electronic information, complementing IR (functional groups), MS (molecular weight), and NMR (structure) for complete analysis.

27. Which electronic transition is symmetry-forbidden but may appear weakly in UV spectra?

a) π → π*

b) n → π*

c) σ → σ*

d) n → σ*

Answer: b) n → π*

Explanation: n → π* transitions are symmetry-forbidden, resulting in low intensity absorption bands, often appearing as shoulders.

28. Why are saturated alkanes not suitable for UV spectroscopic analysis?

a) They absorb too strongly in UV

b) They lack UV-absorbing chromophores

c) They emit UV light instead of absorbing

d) They react with UV radiation

Answer: b) They lack UV-absorbing chromophores

Explanation: Saturated alkanes contain only σ-bonds and lack π-electrons or lone pairs needed for UV absorption in the accessible region.

29. UV absorption primarily affects which part of the molecule?

a) Atomic nuclei

b) Valence electrons

c) Inner shell electrons

d) Molecular rotation

Answer: b) Valence electrons

Explanation: UV photons primarily excite valence electrons involved in bonding, promoting them to higher energy molecular orbitals.

30. Which parameter cannot be directly determined from a UV absorption spectrum?

a) Wavelength of maximum absorption (λmax)

b) Molar absorptivity (ε)

c) Molecular weight

d) Concentration (using Beer-Lambert law)

Answer: c) Molecular weight

Explanation: UV spectra provide absorption and wavelength data but not molecular weight, which requires mass spectrometry or other methods.

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