Question 1

Which region of the electromagnetic spectrum is primarily used in infrared (IR) spectroscopy to study molecular vibrations, and what is the typical range of wavenumbers for this region?

Accepted Answer

Infrared region, 400–4,000 $$cm^{-1}$$

Question 2

A molecule absorbs IR radiation at 1710 $$cm^{-1}. $$Which functional group is most likely present in the molecule?

Accepted Answer

Ketone (C=O)

Question 3

Which of the following statements best explains why the C—H stretching frequency increases as the s-character of the carbon increases?

Accepted Answer

Greater s-character leads to a shorter, stronger bond.

Question 4

A compound shows a strong, broad absorption centered at 3300 $$cm^{-1} in $$its IR spectrum. Which functional group is most likely responsible for this absorption?

Accepted Answer

Alcohol (O—H)

Question 5

Which of the following best describes the effect of conjugation on the IR stretching frequency of a C=O bond?

Accepted Answer

Conjugation decreases the stretching frequency.

Question 6

A mass spectrum of a compound shows two molecular ion peaks (M^+ and M^+2) of equal intensity. Which element is most likely present in the compound?

Accepted Answer

Bromine

Question 7

Which of the following is the correct relationship between wavelength ($$\lambda$$), frequency ($$
u$$), and the speed of light ($$c$$)?

Accepted Answer

$$c = \lambda 
u$$

Question 8

A molecule with a strong, sharp IR absorption at 2250 $$cm^{-1} is $$most likely to contain which functional group?

Accepted Answer

Nitrile (C≡N)

Question 9

In mass spectrometry, what does the base peak represent?

Accepted Answer

The most abundant ion, assigned 100% relative intensity

Question 10

Which of the following best explains why only ions with a positive charge are detected in a mass spectrometer?

Accepted Answer

Only positive ions are deflected by the magnetic field and reach the detector.

Question 11

A compound's IR spectrum shows a strong absorption at 1735 $$cm^{-1}. $$Which functional group is most likely present?

Accepted Answer

Ester

Question 12

Which of the following statements about the fingerprint region in IR spectroscopy is correct?

Accepted Answer

It is unique for each molecule and found between 600 and 1400 $$cm^{-1}.$$

Question 13

Which of the following best describes the effect of increasing atomic mass on bond stretching frequency in IR spectroscopy?

Accepted Answer

Stretching frequency decreases with increasing atomic mass.

Question 14

A molecule shows two sharp N—H stretching absorptions around 3300 $$cm^{-1} in $$its IR spectrum. Which functional group is most likely present?

Accepted Answer

Primary amine ($$RNH_2)$$

Question 15

Which of the following is a limitation of IR spectroscopy?

Accepted Answer

It cannot determine the exact structure of a molecule by itself.

Question 16

A mass spectrum of a compound containing chlorine shows an M^+ peak and an M^+2 peak with a ratio of approximately 3:1. What does this indicate about the isotopic composition of chlorine?

Accepted Answer

Chlorine has two isotopes, with $$^{35}Cl $$being about three times as abundant as $$^{37}Cl.$$

Question 17

Which of the following best describes the process of electron impact ionization in mass spectrometry?

Accepted Answer

A molecule loses an electron due to collision with a high-energy electron, forming a radical cation.

Question 18

Which of the following ions would be detected at m/z = 29 in the mass spectrum of hexane?

Accepted Answer

Ethyl cation

Question 19

A compound's IR spectrum shows a very broad absorption from 2500–3500 $$cm^{-1}$$ and a strong absorption at 1710 $$cm^{-1}. $$Which functional group is most likely present?

Accepted Answer

Carboxylic acid

Infrared Spectroscopy and Mass Spectrometry: Principles and Applications

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