Question 1

Which instrument is shown in the first image, and what is its primary use in organic chemistry?

Accepted Answer

It is an FT-IR spectrometer, used for identifying functional groups via vibrational transitions.

Question 2

Which of the following best describes the principle behind the second instrument shown (mass spectrometer)?

Accepted Answer

It separates ions based on their mass-to-charge ratio ($$m/z) $$after ionization.

Question 3

Refer to the third image. If a photon has a short wavelength, what can you infer about its energy and frequency?

Accepted Answer

It has high energy and high frequency.

Question 4

Which region of the electromagnetic spectrum is primarily used in IR spectroscopy, as shown in the fourth image?

Accepted Answer

Infrared (IR)

Question 5

Given the equation $$\Delta E = E_2$ - E_1$ = h
u$$, what does $$
u$$ represent?

Accepted Answer

The frequency of the absorbed or emitted photon

Question 6

Which molecular process is most directly studied using IR spectroscopy?

Accepted Answer

Molecular vibrations

Question 7

A nonlinear molecule with $$n$$ atoms has how many fundamental vibrational modes?

Accepted Answer

$$3n - 6$$

Question 8

Which of the following bonds is expected to be IR-active?

Accepted Answer

A polar bond that changes dipole moment during vibration

Question 9

The IR absorption frequency for a C–H stretch in an sp hybridized carbon is closest to:

Accepted Answer

$$\sim 3300\ \mathrm{cm}^{-1} ($$sharp)

Question 10

Which functional group is characterized by a strong, sharp IR absorption near $$1700\ \mathrm{cm}^{-1}$$?

Accepted Answer

Carbonyl (C=O)

Question 11

A mass spectrum shows a molecular ion peak at $$m/z = 86$$ and a base peak at $$m/z = 57. $$What does the base peak represent?

Accepted Answer

The most stable and abundant fragment ion

Question 12

Which isotope pattern would you expect for a compound containing two chlorine atoms?

Accepted Answer

Three peaks at M, M+2, and M+4 in a 9:6:1 ratio

Question 13

Which of the following statements about mass spectrometry is TRUE?

Accepted Answer

It separates ions based on their mass-to-charge ratio ($$m/z).$$

Question 14

Which of the following best explains why the IR absorption for a C–D bond occurs at a lower wavenumber than a C–H bond?

Accepted Answer

Deuterium is heavier, increasing the reduced mass ($$\mu) $$and lowering the vibrational frequency.

Question 15

Which of the following is NOT a typical ionization technique in mass spectrometry?

Accepted Answer

Infrared Absorption (IRA)

Question 16

A molecule absorbs IR radiation at $$3300\ \mathrm{cm}^{-1} ($$broad, rounded tip). Which functional group is most likely present?

Accepted Answer

Alcohol (O–H)

Question 17

Which of the following best describes the 'fingerprint region' in an IR spectrum?

Accepted Answer

It is the region between $$600-1400\ \mathrm{cm}^{-1}$$, unique for each molecule.

Question 18

Which of the following statements about IR-active vibrations is correct?

Accepted Answer

Only vibrations that change the dipole moment are IR-active.

Question 19

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

Accepted Answer

Conjugation decreases the bond strength, lowering the force constant ($$k).$$

Question 20

A compound shows a strong IR absorption at $$2249\ \mathrm{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Nitrile (C≡N)

Chapter 12: Infrared (IR) and Mass Spectroscopy – Principles and Applications

Study Guide - Practice Questions