$2850$–$2960$ \text{ cm}^{-1}$

Question 1

Which region of the IR spectrum is commonly referred to as the 'fingerprint region' and why is it important in molecular identification?

Accepted Answer

The region from $1600$ to $400$ cm$^{-1}$; it contains unique absorption patterns for each molecule.

Question 2

What is the relationship between the frequency ($
u$), wavelength ($\lambda$), and speed of light ($c$) for electromagnetic radiation?

Accepted Answer

$c = 
u \lambda$

Question 3

Which factor does NOT affect the position of an IR absorption peak?

Accepted Answer

Concentration of the sample

Question 4

Why do symmetrical alkynes often show weak or absent C≡C stretching absorptions in their IR spectra?

Accepted Answer

Because symmetrical alkynes have a low dipole moment, making the vibration infrared inactive.

Question 5

Which of the following functional groups would show a strong, broad absorption around $3200$–$3400$ cm$^{-1}$ in an IR spectrum?

Accepted Answer

Alcohol (O-H stretch, H-bonded)

Question 6

In mass spectrometry, what does the base peak represent?

Accepted Answer

The peak with the greatest relative abundance in the spectrum

Question 7

A compound contains five carbon atoms. What is the expected relative abundance of the M+1 peak in its mass spectrum due to $^{13}$C?

Accepted Answer

5.5%

Question 8

Which of the following statements about IR absorption intensity is TRUE?

Accepted Answer

Peak intensity increases with the number of similar groups and the dipole moment.

Question 9

What is the energy of a photon with frequency $
u$ according to Planck's equation?

Accepted Answer

$E = h 
u$

Question 10

Which technique is most useful for determining the molecular mass of a compound?

Accepted Answer

Mass spectrometry

Question 11

A mass spectrum of methane shows a small peak at $m/z = 17$. What is the origin of this peak?

Accepted Answer

It is due to the presence of $^{13}$C in the molecule.

Question 12

Which of the following is NOT a requirement for a bond to absorb IR radiation?

Accepted Answer

The bond must be part of a functional group.

Question 13

Which of the following statements about mass spectrometry is FALSE?

Accepted Answer

Mass spectrometry requires large sample amounts for analysis.

Question 14

What is the wavenumber range for C-H stretching vibrations in alkanes?

Accepted Answer

$2850$–$2960$ 	ext{ cm}^{-1}$

Question 15

Which type of mass spectrometry is considered a 'softer' technique and is more likely to show the molecular ion peak?

Accepted Answer

Chemical ionization (CI)

Question 16

What is the effect of increasing the number of fluorine atoms in an alkyl halide on its IR absorption frequency?

Accepted Answer

It increases the absorption frequency.

Question 17

Which of the following is TRUE regarding the nitrogen rule in mass spectrometry?

Accepted Answer

Odd-electron fragments with an odd number of nitrogen atoms have an odd mass.

Question 18

Which of the following is a correct statement about the absorption of IR radiation by a molecule?

Accepted Answer

Absorption occurs only if the energy of the IR photon matches the energy difference between vibrational levels.

Question 19

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

Accepted Answer

The sample is bombarded with high-energy electrons, causing ejection of an electron and formation of a radical cation.

Chapter 13: Introduction to Spectroscopy, Infrared Spectroscopy (IR), and Mass Spectrometry (MS)

Study Guide - Practice Questions