Intro to Spectroscopy (UV-Vis, IR) - Advanced Concepts
Q. In IR spectroscopy, what type of molecular vibrations are typically observed?
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A.
Translational vibrations
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B.
Rotational vibrations
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C.
Stretching and bending vibrations
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D.
Electronic transitions
Solution
IR spectroscopy detects molecular vibrations, specifically stretching and bending of bonds, which occur at characteristic frequencies.
Correct Answer: C — Stretching and bending vibrations
Q. In UV-Vis spectroscopy, what does a peak in the absorption spectrum indicate?
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A.
The presence of impurities
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B.
The concentration of the sample
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C.
A specific electronic transition
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D.
The temperature of the sample
Solution
A peak in the absorption spectrum corresponds to a specific electronic transition of electrons within the molecule.
Correct Answer: C — A specific electronic transition
Q. What is the primary interaction of UV-Vis spectroscopy with a sample?
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A.
Scattering of light
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B.
Absorption of light
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C.
Emission of light
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D.
Reflection of light
Solution
UV-Vis spectroscopy primarily involves the absorption of light by molecules, which promotes electrons to higher energy levels.
Correct Answer: B — Absorption of light
Q. What is the purpose of using a reference sample in UV-Vis spectroscopy?
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A.
To calibrate the instrument
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B.
To increase the sensitivity of the measurement
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C.
To compare the sample's absorbance against a known standard
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D.
To reduce background noise
Solution
A reference sample is used to compare the absorbance of the sample against a known standard, ensuring accurate measurements.
Correct Answer: C — To compare the sample's absorbance against a known standard
Q. What is the role of a solvent in UV-Vis spectroscopy?
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A.
To enhance the absorption of light
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B.
To provide a medium for the sample
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C.
To scatter light for better detection
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D.
To change the wavelength of light
Solution
The solvent serves as a medium for the sample, allowing light to pass through and interact with the analyte without interfering with the measurement.
Correct Answer: B — To provide a medium for the sample
Q. What is the significance of the Beer-Lambert Law in UV-Vis spectroscopy?
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A.
It relates absorbance to concentration and path length.
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B.
It describes the scattering of light in a medium.
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C.
It explains the emission spectra of compounds.
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D.
It determines the molecular weight of a substance.
Solution
The Beer-Lambert Law states that absorbance is directly proportional to the concentration of the absorbing species and the path length of the light.
Correct Answer: A — It relates absorbance to concentration and path length.
Q. What type of detector is commonly used in UV-Vis spectroscopy?
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A.
Photomultiplier tube
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B.
Thermal conductivity detector
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C.
Mass spectrometer
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D.
Fluorescence detector
Solution
Photomultiplier tubes are commonly used as detectors in UV-Vis spectroscopy due to their sensitivity to low light levels.
Correct Answer: A — Photomultiplier tube
Q. Which of the following is a common application of IR spectroscopy?
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A.
Determining the molecular weight of a compound
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B.
Identifying functional groups in organic compounds
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C.
Measuring the pH of a solution
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D.
Analyzing the particle size distribution
Solution
IR spectroscopy is widely used to identify functional groups in organic compounds based on their characteristic absorption bands.
Correct Answer: B — Identifying functional groups in organic compounds
Q. Which of the following statements about IR spectroscopy is true?
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A.
It can only analyze gases.
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B.
It requires a vacuum for accurate measurements.
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C.
It can be used to study solid, liquid, and gas samples.
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D.
It is not effective for polar molecules.
Solution
IR spectroscopy can analyze solid, liquid, and gas samples, making it versatile for various states of matter.
Correct Answer: C — It can be used to study solid, liquid, and gas samples.
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