Organic Chemistry : Laboratory Practices
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Example Question #71 : Laboratory Practices
Why is the mass spectrometry a useful method in laboratory practice?
Two of these answers.
It helps determine the mass of a molecule, and gives clues about its structures by showing the most favored molecular 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.
It can be used in tandem with gas chromatography to determine the molecular size of the compound.
It helps determine the mass of a molecule, though it cannot provide any other structural information.
Two of these answers.
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 #71 : Laboratory Practices
What makes an amine peak different than an alcohol peak, in infrared spectra?
Amine peaks show up in the 
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 
There is no way to distinguish amine peaks from alcohol peaks.
Amine peaks show up in the 
Amine peaks show up in the
The correct answer is "Amine peaks show up in the 
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