Infrared Spectroscopy

This is an MCQ-based quiz for GRE on the topic of Infrared Spectroscopy.

The measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms.

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Which of the following statements is true concerning infrared spectroscopy?

IR spectroscopy is useful in determining the size and shape of a compound's carbon skeleton. An IR spectrometer shines infrared light on a compound and records the positions where the light is blocked by the compound. This results in the spectrum's peaks. When the infrared light frequency matches the frequency of bond vibration in a molecule, a peak is recorded on the spectrum. Functional groups can be identified by looking in the fingerprint region of the spectrum.

Which of the following statements is true concerning infrared (IR) spectroscopy?

IR spectroscopy is used to determine the frequency of vibrations between atoms The fingerprint region is most easily used to determine the functional groups in the molecule IR spectroscopy is used to determine the shape of the carbon backbone IR spectroscopy can be used to easily determine molecular mass

In IR spectroscopy, the vibration between atoms is caused by which of the following?

The number of protons in a nucleus The movement of electrons to higher energy levels The overall molecular weight of the molecule Dipole moments between atoms

Which of the following functional groups exhibits the highest frequency in an infrared (IR) spectrum?

Ketone Ester Aldehyde Alcohol Nitrile

An alcohol group in a compound would result in a broad dip around what part of the infrared (IR) spectrum?

1200cm-1 1700cm-1 3400cm-1 2800cm-1

An unknown compound is analyzed using infrared spectroscopy. A strong, sharp peak is observed at a frequency of 1750cm-1. What functional group is present?

Alcohol Ester Nitrile Unsaturated ketone Saturated ketone

Approximately where would a carbonyl peak be found on an IR spectrum?

The peak location will vary depending on the compound being analyzed. 1700cm-1 1000cm-1 2800cm-1

An IR spectrum reading is taken before and after treating acetone with the reducing agent NaBH4. What IR peak readings would be seen for the reactant acetone and for the predicted product?

Acetone: 1750cm−1, Product: 1000cm−1

Acetone: 1000cm−1, Product: 3300cm−1

Acetone: 1750cm−1, Product: 3300cm−1

Acetone: 3300cm−1, Product: 3300cm−1

Acetone: 3300cm−1, Product: 1750cm−1

After taking an IR spectrum of a sample synthesized in the lab, you have 3 IR peaks. Peak A has a 70 transmittance, peak B has a 50 transmittance, and peak C has a 5 transmittance. Which peak has the greatest absorbance?

Peak B

Peak A

Peak C

All the peaks have the same transmittance

Peaks A and B

What is the absorbance of an IR peak with a 25% transmittance?

2.50

0.81

0.60

0.33

1.04

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