DETERMINING
CHEMICAL
FORMULAS
What if you discovered the cure for cancer?
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Any new chemical that is discovered that is a potential health
care treatment must have its molecular formula submitted to the
Ministry of Health.
How do you determine the molecular formula of some new
compound that neither your nor anyone may have never seen
before?
It’s actually not that hard…all you need is a device called a
mass spectrometer and some chemistry fundamentals – some of
which are hundreds of years old!
The Mass Spectrometer
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A mass spectrometer is a
lab instrument that
measures the molar mass
of a compound.
The machine can also give
you a reading of each
element in the compound
based on mass and
percentage.
Look at this reading from the
Cassini Spacecraft as it flew by
one of Saturn’s moons –
Enceladus.
Law of Constant Composition
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A compound contains elements in certain fixed
proportions (ratios) and in no other
combinations.
This law is important in formula determination
because it basically says that each compound
has only one formula…if you find something
that matches a known formula then that is
what you have found – no arguments.
Water will always be H2O…Find that and you have
found water.
Empirical vs. Molecular
Before you can determine the molecular
formula of a compound you must first
determine the empirical formula.
 An empirical formula is the lowest whole
number ratio of elements in a compound.
 Think of empirical as a formula in lowest
terms.
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The molecular formula of a compound is
the actual number of atoms in one
molecule of a compound.
Determining Empirical Formulas
In order to determine the empirical formula
for a compound you need the percent
composition for it…
 The mass spectrometer will give you
this…or the question will tell you!
 Let’s try an example of a problem in which
we must determine the percent
composition.
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Sample Problem 1: The substance that gives sour milk that
lovely taste is lactic acid. It consists of 40.00% Carbon and
6.71% Hydrogen with the remainder being oxygen. Determine
its empirical formula…Or Else!!!
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C = 40.00g ÷ 12.01g/mol = 3.33mol
C = 3.33mol ÷ 3.33mol = 1
H = 6.71g ÷ 1.01g/mol = 6.64mol
H = 6.64mol ÷ 3.33mol = 2
O = 53.29g ÷ 16.00g/mol = 3.33mol
O = 3.33mol ÷ 3.33mol = 1
Therefore the empirical formula for
lactic acid is CH2O.
Steps to Solve
 Assume 100g
(%  g)
 g  mol
(÷ by g/mol)
 Divide by
lowest mol
Empirical Is Not Enough!
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If you look at this slideshow carefully – you will have noticed
both glucose (C6H12O6) and lactic acid (C3H6O3) have the same
empirical formula – but they are very different molecules it
terms of properties and function within the human body!
Many molecules can have the same empirical
formulas so it is absolutely necessary to be able
to determine the molecular formula for any
compound.
In order to determine the molecular formula we
need two pieces of information:
 Empirical Formula (or a way to get it…like
percent composition…a la the mass
spectrometer)
 Molar mass of the compound (The almighty
mass spectrometer will give us this too!)
Behold!...The Mass Spec!
Sample Problem 2: Determine the molecular formula of a
compound containing 85.7% carbon and 14.3% hydrogen by mass.
The molar mass of the compound is 84g/mol. Solve…Or Else!!!
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We do not have the empirical formula but we do have a way to get it.
(by using the Percent Composition)!
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C = 85.7g ÷ 12.01g/mol = 7.136mol ÷ 7.136mol = 1
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H = 14.3g ÷ 1.01g/mol = 14.16mol ÷ 7.136mol = 2
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Therefore the empirical formula is CH2.
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Molar Mass
Empirical Formula Mass
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This means that the molecular formula is 6 times larger than the
empirical formula so we need to multiply the empirical formula by 6 to
get the molecular formula.
6 x CH2 = C6H12
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The Molecular formula is C6H12.
= 84g/mol
=6
14.03g/mol
Sample Problem 3: Determine the molecular formula of a compound
with the empirical formula P2O3 and a molar mass of 220.0g/mol.
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We are given the empirical formula
outright so you go directly to the second
step of the solving process.
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Molar Mass
= 220.0g/mol = 2
Empirical Formula Mass
109.94g/mol
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This means that the molecular formula is 2
times larger than the empirical formula so
we need to multiply the empirical formula
That’s All I Got!...
“I know it’s a lot to think about…
So I will leave you to it.”