MCQ in Principle of UV spectroscopy

Here is the complete set of 30 MCQ test quiz questions on the Principle of UV Spectroscopy, including answers and explanations: ```html

MCQ Quiz: Principle of UV Spectroscopy

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.

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.

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, showing absorbance proportional to concentration and path length.

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

a) Emission constant
b) Extinction coefficient
c) Excitation energy
d) Electron density

Answer: b) Extinction coefficient

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

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.

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 energy from photons promotes electrons to higher molecular orbitals.

7. Which transition has the highest energy?

a) n → π*
b) π → π*
c) σ → σ*
d) n → σ*

Answer: c) σ → σ*

Explanation: Breaking σ-bond requires high energy, so σ → σ* transition occurs in deep UV end.

8. Which transition is 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 lone pair electrons undergo n → π* transition.

9. Chromophores are defined as:

a) Atoms absorbing infrared radiation
b) Part of molecule responsible for color/UV absorption
c) Heavy metal ions
d) Aromatic solvent groups

Answer: b) Part of molecule responsible for color/UV absorption

Explanation: Chromophores are molecular moieties absorbing UV-visible radiation responsible for color.

10. Which group is an auxochrome?

a) Nitro group
b) Amine group
c) Carbonyl group
d) Phenyl

Answer: b) Amine group

Explanation: Auxochromes like -OH, -NH2 intensify absorption of chromophores by resonance and polarity 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 excites electrons from bonding or non-bonding orbitals to antibonding orbitals.

12. Which of the following 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 polarity, temperature, and pH can cause shifts in λmax by altering electronic environment.

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

a) Hypsochromic shift
b) Bathochromic shift
c) Hypochromic effect
d) Hyperchromic effect

Answer: b) Bathochromic shift

Explanation: Bathochromic shift is a red shift, indicating absorption at longer wavelength due to auxochrome effect.

14. The decrease in absorption intensity without wavelength shift 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.

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

a) Water
b) Hexane
c) Ethanol
d) Cyclohexane

Answer: c) Ethanol

Explanation: Ethanol has low UV cutoff and minimal absorbance interfering with UV spectra.

16. What type of electronic transition corresponds to absorption by aromatic compounds?

a) n → π*
b) π → π*
c) σ → σ*
d) d-d transitions

Answer: b) π → π*

Explanation: Aromatic compounds have conjugated π systems leading to π → π* transitions in UV.

17. What instrument component ensures selection of a narrow wavelength band in UV spectrophotometer?

a) Monochromator
b) Photomultiplier tube
c) Light source
d) Sample holder

Answer: a) Monochromator

Explanation: The monochromator selects individual wavelengths from the polychromatic UV light source.

18. In UV spectrophotometry, what is measured to obtain absorbance?

a) Transmitted light intensity
b) Reflected light intensity
c) Emitted light intensity
d) Scattered light intensity

Answer: a) Transmitted light intensity

Explanation: Absorbance is calculated from intensity of light before and after passing through the sample.

19. Which of the following is NOT a chromophore?

a) C=C double bond
b) Carbonyl group
c) Alkane chain
d) Benzene ring

Answer: c) Alkane chain

Explanation: Alkanes without double bonds do not absorb UV light significantly; hence, no chromophore.

20. Which of the following causes a hypsochromic shift?

a) Increase in conjugation
b) Addition of electron withdrawing groups
c) Increase in solvent polarity
d) None of the above

Answer: b) Addition of electron withdrawing groups

Explanation: Electron withdrawing groups can shift absorption to shorter wavelengths (blue shift).

21. The term "hyperchromic effect" refers to:

a) Increase in absorbance
b) Shift to shorter wavelength
c) Shift to longer wavelength
d) Decrease in absorbance

Answer: a) Increase in absorbance

Explanation: Hyperchromic effect refers to increased absorption intensity in the UV spectrum.

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 absorbance reading
d) To act as a reference peak

Answer: b) To set zero absorbance baseline

Explanation: The blank corrects for solvent absorbance and sets baseline for sample measurements.

23. Which factor does NOT affect the absorbance in UV spectroscopy?

a) Sample concentration
b) Path length
c) Instrument beam width
d) Atmospheric pressure

Answer: d) Atmospheric pressure

Explanation: Atmospheric pressure does not influence absorbance, unlike concentration and path length.

24. When two absorbance maxima are observed due to conjugation, what is this called?

a) Hyperchromic effect
b) Vibronic coupling
c) Auxochrome effect
d) Spectral splitting

Answer: b) Vibronic coupling

Explanation: Vibronic coupling results when vibrational and electronic transitions combine causing multiple peaks.

25. Which one is the UV cutoff for quartz cuvettes commonly used in UV spectroscopy?

a) 220 nm
b) 330 nm
c) 400 nm
d) 190 nm

Answer: d) 190 nm

Explanation: Quartz cuvettes transmit UV light down to about 190 nm, suitable for UV spectroscopy.

26. UV-visible spectroscopy is often combined with which technique to obtain structural information?

a) Infrared spectroscopy
b) Mass spectrometry
c) NMR spectroscopy
d) All of the above

Answer: d) All of the above

Explanation: UV-Vis is complementary to IR, MS, and NMR for complete molecular characterization.

27. Which electronic transition is forbidden by symmetry rules but can appear weakly in UV spectra?

a) π → π*
b) n → π*
c) σ → σ*
d) d-d transition

Answer: b) n → π*

Explanation: n → π* transitions are symmetry forbidden, often less intense and appear as shoulder peaks.

28. Why is UV spectroscopy less suitable for alkanes?

a) Alkanes absorb strongly in UV
b) Alkanes lack chromophores for UV absorption
c) Alkanes emit UV light
d) Alkanes react with solvents in UV

Answer: b) Alkanes lack chromophores for UV absorption

Explanation: Saturated alkanes do not have conjugated electrons to absorb UV radiation effectively.

29. UV absorption leads to molecular changes primarily in which part of the molecule?

a) Nuclei
b) Electrons
c) Atoms
d) Ions

Answer: b) Electrons

Explanation: UV light causes excitation of electrons to higher energy orbitals without affecting nuclei directly.

30. Which parameter is NOT directly obtained from a UV absorption spectrum?

a) Wavelength of maximum absorption
b) Absorbance at λmax
c) Molecular weight
d) Concentration of analyte

Answer: c) Molecular weight

Explanation: UV spectra provide absorbance and wavelength data, but molecular weight requires other techniques.

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