Obtaining Molecular Formulae

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Obtaining Molecular Formulae from Combustion
Analysis and/or Mass Spectrometry
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Elemental (C,H,N) Analysis
Relative abundance of elements in a compound
given in weight percent (wt) Notice that C H
N do not add up to 100
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Elemental (C,H,N) Analysis
Since the elements are determined by combustion
in the presence of excess oxygen, it is difficult
to get an accurate determination for this element
in combustion analysis. Therefore, assume that
oxygen is responsible for any unaccounted mass
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Elemental (C,H,N) Analysis
Inasmuch as we are interested in the molecular
formula, which is a molar relationship, we need
to convert weight percent into mole percent. We
start by bringing in the atomic weight.
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Elemental (C,H,N) Analysis
wt at. wt. c
Dividing each mass by its corresponding atomic
weight gives a measure of molar contribution (c).
These numbers mean nothing on an absolute scale
therefore, we need to normalize them.
c

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Elemental (C,H,N) Analysis
(cE cT) x 100 mol
c
cT
Relating each individual molar contribution to
the total of all molar contributions gives a
measure of molar percent (mol).
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Elemental (C,H,N) Analysis
c
smallest mol
C7H13NO but also C14H26N2O2, etc.
Dividing all mol figures by the smallest mol
will give us an idea of the molar ratio of the
elements. Keep in mind that this is just a
ratio, not the molecular formula!
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Mass Spectrometry
Mass spectrometry gives us an idea of the
molecular weight of a compound, whichcombined
with the molar ratiocan inform us of the
molecular formula
Molar ratios C7H13NO C14H26N2O2 C21H39N3O3 C28H52N
4O4 C35H65N5O5
Mass spec MW 127
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Mass Spectrometry
Alternatively, high-resolution mass spectrometry
can often provide a pretty good indicator of
molecular formula directly. This is because each
unique combination of atoms gives a slightly
different total molecular weight.
HRMS MW 127.0997
C7H13NO