Questions and answers for exam Preparation

MCQs

Total Questions : 147 | Page 12 of 15 pages

Question 111. Why are rotational transitions of little use to a spectroscopist?

Because the energy required to induce a rotational transition is so small that it cannot be measured
Because, in liquids and solids, spectral lines corresponding to rotational transitions are broadened as the result of molecular collisions and other interactions
Because rotational transitions are extremely rare
All of these

Discuss Question

Answer: Option B. -> Because, in liquids and solids, spectral lines corresponding to rotational transitions are broadened as the result of molecular collisions and other interactions

Question 112. A simple harmonic oscillator may absorb energy

when the amplitudes are the same.
at no time.
when the frequencies match exactly
at anytime.

Discuss Question

Answer: Option C. -> when the frequencies match exactly

Question 113. Vibrational spectroscopy is

a large mass on a weak spring
a class of spectroscopic techniques which analyzes molecular motions
a flashlight through a prism and shake it
an Infrared spectroscopy

Discuss Question

Answer: Option B. -> a class of spectroscopic techniques which analyzes molecular motions

Question 114. Which of the following components of a monochromator is the dispersing element?

None of these
The collimating lens
The diffraction grating
The entrance slit

Discuss Question

Answer: Option C. -> The diffraction grating

Question 115. UV-Vis spectroscopy of organic compounds is usually concerned with which electronic transition(s)?

n → σ*
none of these
n → π* and π → π*
σ → σ*

Discuss Question

Answer: Option C. -> n → π* and π → π*

Question 116. Beer's Law states that

absorbance is proportional to both the path length and concentration of the absorbing species
None of these
absorbance is proportional to the log of the concentration of the absorbing species
absorbance is equal to P0 / P

Discuss Question

Answer: Option A. -> absorbance is proportional to both the path length and concentration of the absorbing species

Question 117. Why do fluorescence spectrometers often use double-beam optics?

So a reference solution can be used
To compensate for beam attenuation by the monochromator
To compensate for power fluctuations in the radiation source
All of these

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Answer: Option C. -> To compensate for power fluctuations in the radiation source

Question 118. For a molecule to absorb IR, why must the molecule's vibrations cause fluctuations in the dipole moment of the molecule?

Because for absorption to occur, the radiation must interact with the electric field caused by changing dipole moment
Because fluctuations in the dipole moment allow the molecule to deform by bending and stretching
All of these
Because a change in dipole moment lowers the energy required for electronic transitions

Discuss Question

Answer: Option A. -> Because for absorption to occur, the radiation must interact with the electric field caused by changing dipole moment

Question 119. In the intersystem crossing

the spin of an excited electron reverses, changing the state of the molecule (from singlet state to triplet state or vice versa)
All of these
a molecule converts excess electronic energy to vibrational energy
a molecule converts its excess energy to light, and emits a photon

Discuss Question

Answer: Option A. -> the spin of an excited electron reverses, changing the state of the molecule (from singlet state to triplet state or vice versa)

Question 120. Why must the radiation source for fluorescence spectrometry be more powerful than for absorption spectroscopy?

Because the sample won't fluoresce if the incident radiation is of low power
To allow for scattering by the sample
None of these
Because the magnitude of the output signal is proportional to the power of the incident radiation

Discuss Question

Answer: Option D. -> Because the magnitude of the output signal is proportional to the power of the incident radiation