3/15/2017
Molecular Formulas
molecular formula of a
compound is either:
 The
Molecular Formulas
same as its empirical formula
 Is an integer multiple of its
empirical formula
 The
Molecular Formulas
Molecular Formulas
 Once
you have determined the
empirical formula of a compound,
you can determine its molecular
formula if you know the compound’s
molar mass
 The molar mass of a compound can
be determined by using an instrument
called a mass spectrometer
Molecular Formulas
a mass spectrometer, a
compound is broken into ions that
are then accelerated by an electric
field
 A magnetic field of known
strength deflects the ion beam
from its straight-line path
 In
Molecular Formulas
 The
mass of the compound can be
calculated by how far the ion
beams path has been changed by
the magnetic field
 Low
mass ions: deflected the most
 High mass ions: deflected the least
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3/15/2017
Molecular Formulas
Problem
 An
experimental compound is
known to contain:
 carbon: 92.31% (by mass), and
 hydrogen: 7.69% (by mass)
Problem
molar mass of the compound
has been determined to be
26 g/mol using a mass spec
 What is the molecular formula of
the unknown compound?
 The
Solution
Solution
 Assume
that we have 100 g of the
unknown compound. Therefore:
 92.31%
C = 92.31g C
 7.69% H = 7.69 g H
 Calculate
the empirical formula
 C7.69H7.69
→ C1H1 = CH
 Calculate the mm of the EF:
 13
g/mol
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Molecular Formulas
Solution
can now derive the molecular
formula by multiplying the
empirical formula (CH) by the
ratio of their molar masses:
 We
 We
can see that the unknown
compound contains exactly 2x as
many C and H atoms as the EF
 (2)(CH)
= C2H2
correct molecular formula is
C2H2, which is ethyne
 The
Problem
 Calculate
the molecular formula
of a compound whose molar mass
is 60.0 g/mol and whose empirical
formula is CH4N
Solution
 Known:
 Empirical
formula: CH4N
 MMM = 60.0 g/mol
 Unknown:
 Molecular
formula: C?H?N?
 Strategy:
 (MFmm/EFmm)(EF)
Solution
 Calculate
 CH4N
= 30 g/mol
 Calculate
=
MFmm/EFmm:
(60 g/mol)/(30 g/mol) = 2
 Calculate
=
the EFmm:
MF:
Solution
 Check
yourself:
 Does the molecular formula have
the molar mass stated in the
problem (60 g/mol)?
 Yes
(2)(CH4N) = C2H8N2
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