Q: Which IR absorption would you expect to be strongest and broadest for a carboxylic acid?

O–H stretch at $$3300$$–$$2500\ \text{cm}^{-1}$$

Q: Which of the following bonds would absorb at the highest wavenumber in an IR spectrum?

C–H bond in an alkyne ($$\approx 3300\ \text{cm}^{-1})$$

Q: Which equation correctly relates the energy, frequency, and wavelength of electromagnetic radiation?

$$E = h\nu = \dfrac{hc}{\lambda}$$

Question 1

Which of the following statements best explains why the base peak in a mass spectrum is not always the molecular ion peak?

Accepted Answer

The base peak corresponds to the most stable and abundant fragment ion, not necessarily the molecular ion.

Question 2

A compound with the formula $$C_5H_{12}$$ shows a molecular ion peak at $$m/z = 72$$ and a base peak at $$m/z = 57 in $$its mass spectrum. Which structural feature does this suggest?

Accepted Answer

The compound is isopentane, and the base peak at $$m/z = 57 is $$due to the formation of a secondary carbocation.

Question 3

Which of the following best describes the effect of resonance on the IR absorption frequency of a carbonyl group?

Accepted Answer

Resonance electron donation decreases the absorption frequency by increasing single-bond character.

Question 4

A mass spectrum of a compound shows two peaks of nearly equal intensity at $$m/z = 120$$ and $$m/z = 122. $$What does this indicate about the compound?

Accepted Answer

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

Question 5

Which IR absorption would you expect to be strongest and broadest for a carboxylic acid?

Accepted Answer

O–H stretch at $$3300$$–$$2500\ 	ext{cm}^{-1}$$

Question 6

Which of the following bonds would absorb at the highest wavenumber in an IR spectrum?

Accepted Answer

C–H bond in an alkyne ($$\approx 3300\ 	ext{cm}^{-1})$$

Question 7

A compound gives a strong IR absorption at $$1740\ 	ext{cm}^{-1}$$ and another at $$1250\ 	ext{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Ester

Question 8

Which of the following statements about UV/Vis spectroscopy is correct?

Accepted Answer

The more conjugated double bonds a molecule has, the longer the wavelength ($$\lambda_{max}) of $$maximum absorption.

Question 9

Which equation correctly relates the energy, frequency, and wavelength of electromagnetic radiation?

Accepted Answer

$$E = h
u = \dfrac{hc}{\lambda}$$

Question 10

A sample analyzed by GC-MS shows two peaks with identical mass spectra but different retention times. What does this indicate?

Accepted Answer

The sample contains two structural isomers.

Question 11

Which of the following best explains why the IR absorption of a C–O bond in an alcohol is different from that in an ester?

Accepted Answer

The C–O bond in an ester has partial double-bond character due to resonance, shifting its absorption to a higher wavenumber.

Question 12

A mass spectrum of a compound shows a molecular ion peak at $$m/z = 78$$ and a significant M+2 peak at $$m/z = 80. $$What is the most likely explanation?

Accepted Answer

The compound contains a chlorine atom, which has two isotopes ($$^{35}Cl $$and $$^{37}Cl) $$with a 3:1 abundance ratio.

Question 13

Which of the following is a correct application of the Beer–Lambert Law in UV/Vis spectroscopy?

Accepted Answer

The absorbance ($$A) of a $$solution is directly proportional to the concentration ($$c) of $$the absorbing species, the path length ($$l$$), and the molar absorptivity ($$\varepsilon$$): $$A = \varepsilon c l$$

Question 14

Which of the following bonds is expected to have the lowest IR stretching frequency?

Accepted Answer

C–Cl single bond ($$\approx 700\ 	ext{cm}^{-1})$$

Question 15

Which of the following is NOT a requirement for a molecular vibration to be IR active?

Accepted Answer

The vibration must cause a change in the molecular mass.

Question 16

Which of the following best describes the effect of an auxochrome on a chromophore in UV/Vis spectroscopy?

Accepted Answer

An auxochrome shifts the absorption maximum and increases the intensity of the absorption.

Question 17

Which of the following is a real-world application of UV/Vis spectroscopy?

Accepted Answer

Measuring the rate of a chemical reaction by monitoring the change in absorbance over time.

Chapter 13: Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy – Study Notes

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