Question 1

Which of the following statements best describes the relationship between frequency and wavelength in the electromagnetic spectrum?

Accepted Answer

Frequency and wavelength are inversely proportional, as shown by $$c = \lambda 
u.$$

Question 2

A molecule shows a strong, sharp IR absorption at 1718 cm$$^{-1}$$ and no significant absorption near 3300 cm$$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Ketone

Question 3

Which IR absorption would you expect for a primary amine (RNH$$_2$$)?

Accepted Answer

A broad absorption with two sharp spikes around 3300 cm$$^{-1}$$

Question 4

Which of the following best explains why conjugation lowers the IR stretching frequency of a C=C bond?

Accepted Answer

Conjugation allows for delocalization of electrons, reducing the bond order and lowering the stretching frequency.

Question 5

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

Accepted Answer

Carboxylic acid

Question 6

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

Accepted Answer

It is found between 600 and 1400 cm$$^{-1}$$ and contains complex vibrations unique to each molecule.

Question 7

Which equation correctly relates the energy of a photon to its frequency?

Accepted Answer

$$E = h 
u$$

Question 8

A mass spectrum of a compound containing bromine shows two molecular ion peaks (M and M+2) of equal height. What does this indicate?

Accepted Answer

The compound contains bromine, which has two isotopes of nearly equal abundance.

Question 9

Which of the following IR absorptions is characteristic of a nitrile group?

Accepted Answer

A sharp absorption around 2200–2300 cm$$^{-1}$$

Question 10

Which of the following statements about mass spectrometry is correct?

Accepted Answer

Mass spectrometry fragments the molecule and measures the mass of the fragments.

Question 11

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

Accepted Answer

Ester

Question 12

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

Accepted Answer

Frequency decreases with increasing atomic mass.

Question 13

Which type of molecular ion is detected in mass spectrometry after electron impact ionization?

Accepted Answer

Radical cations

Question 14

Which IR absorption is most likely to be observed for an sp-hybridized C–H bond?

Accepted Answer

3300 cm$$^{-1} ($$sharp)

Question 15

A mass spectrum of a compound containing chlorine shows an M peak three times higher than the M+2 peak. What does this indicate?

Accepted Answer

The compound contains chlorine, with $$^{35}Cl (75.5$$%) and $$^{37}Cl (24.5$$%) isotopes.

Question 16

Which of the following is a limitation of IR spectroscopy?

Accepted Answer

Some signals may be ambiguous, and IR alone cannot determine a structure.

Question 17

Which of the following best describes the base peak in a mass spectrum?

Accepted Answer

The tallest peak, assigned an abundance of 100%.

Question 18

Which IR absorption is most characteristic of an amide carbonyl group?

Accepted Answer

1640–1680 cm$$^{-1}$$

Question 19

Which of the following is true about the FT-IR spectrometer compared to a traditional IR spectrometer?

Accepted Answer

FT-IR completes scans faster and averages multiple scans for better sensitivity.

Question 20

A molecule’s IR spectrum shows a strong absorption at 1660 cm$$^{-1}$$ and a weaker absorption at 1600 cm$$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Aromatic ring

Infrared Spectroscopy and Mass Spectrometry: Structure Determination in Organic Chemistry

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