Organic Chemistry : Using Other Organic Chemistry Lab Techniques

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #71 : Laboratory Practices

Why is the mass spectrometry a useful method in laboratory practice?

Possible Answers:

It helps show the approximate mass of a molecule, though it is unlikely to at all show the actual mass of the original unfragmented molecule.

It helps determine the mass of a molecule, and gives clues about its structures by showing the most favored molecular fragments.

It can be used in tandem with gas chromatography to determine the molecular size of the compound.

Two of these answers.

It helps determine the mass of a molecule, though it cannot provide any other structural information.

Correct answer:

Two of these answers.

Explanation:

The correct answer is "two of these" -- namely, "It can be used in tandem with gas chromatography to determine the molecular size of the compound" and "It helps determine the mass of a molecule, and gives clues about its structures by showing the most favored molecular fragments."

Gas chromatography (GC) helps show the molecular weight of compounds in a sample, by illuminating their various evaporation rates (which correspond to molecular size). Mass spec helps show the mass of the molecule - to an exact amu - by ionizing it to a positive cation and putting it into an electrical field and detector which can record its charge and mass. Mass spec also illuminates structural components of the molecule, because it shows the masses of the most fragments, and the likeliest fragments occur according to the structural possibility of the original molecule to fragment in a particular way.

Incorrect answers:

"It helps determine the mass of a molecule, though it cannot provide any other structural information." -- It shows structural information by showing the massses of the fragments.

"It helps show the approximate mass of a molecule, though it is unlikely to at all show the actual mass of the original unfragmented molecule." -- mass spectrometry shows the actual mass of the unfragmented molecule as the "parent peak" -- the peak with the largest m/z value (even if lower abundance than the base peak, which usually corresponds to the most likely fragmentation pattern).

Example Question #72 : Organic Chemistry

What makes an amine peak different than an alcohol peak, in infrared spectra?

Possible Answers:

Amine and alcohol peaks show up in exactly the same range, but amine peaks are strong and broad compared to alcohol peaks.

Amine peaks show up in the  range, whereas alcohol peaks show up in the  range.  Amine peaks are sharper, "pointier," than alcohol peaks, which are usually strong and broad.

Amine peaks show up in the  range, whereas alcohol peaks show up in the  range. Amine peaks are sharper, "pointier," than alcohol peaks, which are usually strong and broad.

There is no way to distinguish amine peaks from alcohol peaks.

Amine peaks show up in the  range, whereas alcohol peaks show up in the  range.  Amine peaks are strong and broad, whereas alcohol peaks are strong but sharp. 

Correct answer:

Amine peaks show up in the  range, whereas alcohol peaks show up in the  range. Amine peaks are sharper, "pointier," than alcohol peaks, which are usually strong and broad.

Explanation:

The correct answer is "Amine peaks show up in the  range, whereas alcohol peaks show up in the  range. Amine peaks are sharper, "pointier," than alcohol peaks, which are usually strong and broad." The other answers contradict this statement.

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