Working with
Empirical Formulas,
and Molecular Formulas
loosely based on Chap 3 Sec 2, 3 & 4
of Jespersen 7th ed
Dr. C. Yau
Spring 2015
1
Chemical Formula:
Particulate vs. macroscopic levels
When we see the formula H2S…
at the particulate level we should be
thinking of one molecule of H2S with
2 H atoms and 1 S atom,
at the macroscopic level we should be
thinking of one mol H2S with
2 mol H and 1 mol S.
2
Example 3.5 p. 116
Calcium phosphate is widely found in nature in
the form of natural minerals. It is also found in
bones and some kidney stones.
In many instances if we determine one element in
a compound we can find out how much of the
compound is present.
In one case a sample is found to contain 0.864
mole of phosphorus. How many moles of
calcium phosphate will that represent.
Hint: Do we need Avogadro's number?
Do Practice Exercises 3.7, 3.8 & 3.9 on p.116
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Example 3.6 p.117
Chlorophyll, the green pigment in leaves,
has the formula C55H72MgN4O5.
If 0.0011 g of Mg is available to a plant cell
for chlorophyll synthesis, how many
grams of carbon will be required to
completely use up the magnesium?
Do we need Avogadro’s number?
Do Practice Exercises 10, 11 on p.118
4
Percent Composition
LEARN THIS!
"Percent composition" refers to the % of
each element in a compound by mass.
For example, the % composition of CO2 is
27.29 % C and 72.71 % O.
How do we come up with these
numbers?
(Unless specified otherwise, please
always give answer to 4 sig. fig.)
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Example 3.7 p. 118
A sample of a liquid with a mass of 8.657 g
was decomposed into its elements and
gave 5.217 g of carbon, 0.9620 g of
hydrogen and 2.478 g of oxygen. What is
the percentage composition of this
compound?
Remember: To find percent, always think..
 part 

 x 100
 whole 


Do Practice Exercises 12 & 13 p.119
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Example 3.8 p. 119
Do the mass percentages of 25.94% N and
74.06% O match the formula N2O5?
Do Practice Exercises 14 & 15 p.120
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Empirical Formulas
The word "empirical" means "experimental".
"Empirical formula" refers to a formula
based on experimental evidence (rather
than theory).
If we base a formula on experimental
evidence, we can only come up with the
elements in the lowest ratio.
This is why students commonly mistake the
meaning of "empirical" to mean "simplest
ratio."
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Empirical Formulas
When we write a formula, whether it is an
empirical formula or molecular formula,
remember that the subscripts refer to the
number atoms (or moles of atoms).
THEY DO NOT REFER TO GRAMS!
So when we see H2S, we are not thinking of 2
grams of H and 1 g of S, but
2 atoms S and 1 atom S
2 mol H and 1 mol S.
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Empirical Formula
Example 3.9 p. 121
A 2.57 g sample of a compound composed
of only tin and chlorine was found to
contain 1.17 g of tin. what is the
compound's empirical formula?
Hint: We need a ratio of moles not grams!
I STRONGLY recommend you set up the problem the way I am going
to show you.
You will be penalized if you have any "run-on" statements in your
setup! My setup will help you avoid such "run-on" statements.
Do practice exercises 3.16 & 3.17 p.122.
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Empirical Formula
Example p.122
One of the compounds of iron and oxygen,
"black iron oxide," occurs naturally in the
mineral magnetite.
When a 2.448 g sample was analyzed it was
found to have 1.771 g of Fe. Calculate the
empirical formula of this compound.
Do Practice Exercises 18 & 19 p.123
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Empirical Formula
Example 3.10 p.123
A white powder used in paints, enamels, and
ceramics has the following percentage
composition: Ba, 69.6%; C, 6.09%; and O, 24.3%.
What is its empirical formula? What is the name of
this compound?
Do practice exercises 20 & 21 p.124.
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Empirical Formula
Example 3.11 p.126
A 0.5438 g sample of a liquid
consisting of only C, H, and O was
burned in pure oxygen, and 1.039 g
of CO2 and 0.6369 g of H2O were
obtained. What is the empirical
formula of the compound.
Do Practice Exercises 22 & 23 p.127.
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From Empirical Formula
to Molecular Formula
Remember that ionic compounds do NOT have
"molecular" formula! So, we are dealing with
only molecular compounds here.
Ionic compounds have ONLY empirical
formulas (subscripts are in the lowest ratio).
To determine the molecular formula, you must
be given one extra piece of information:
the molecular weight: weight of one molecule
or weight of one mole of molecules.
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Difference between Empirical Formula
and Molecular Formula
Hydrazine used in rocket fuel has the
molecular formula of N2H4.
It tells us that each molecule has 2 N atoms
and 4 H atoms. It is NOT just a ratio.
H
H Its MW is 2(14.01) + 4(1.008)
N N
= 32.05 u/molecule
H
H or 32.05 g/mol
Its empirical formula is N1H2, usually written
simply as NH2.
Its empirical weight (EW) is 16.03 u/f.u. or
16.03 g/mol
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The Use of “f.u.”
• Previously you learned to use “f.u.” in
place of “molecule” when dealing with an
ionic compound.
• The smallest unit in an ionic compound is
indeed its “f.u.”
• However, it does not mean “f.u.” is limited
to only ionic compounds.
• For molecular compounds, if the molecular
formula is reduced to its lowest ratio, it is
also referred to as a “f.u.”
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The Use of “f.u.”
Thus, “f.u.” is used for any substance when
shown in the simplest ratio.
IONIC COMPOUND:
It is always referred to as a f.u. (e.g. Na2S)
MOLECULAR COMPOUND:
• If formula shows true # of atoms of each
element (e.g. N2H4), it is a molecule.
• If formula shows only the lowest ratio of
atoms of each element (e.g. NH2) it is a f.u.
N2H4 is the molecular formula.
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NH2 is the empirical formula.
Empirical Formula vs Molecular Formula
• N2H4 is the molecular formula
MW = 32.05 u/molecule or 32.05g/mol
• NH2 is the empirical formula
EW = 16.03 u/f.u. or 16.03 g/mol
(EW = empirical weight)
Note: The MW is a multiple of its EW.
Divide MW by EW to determine which
multiple it is and multiply the empirical
formula by this multiple to get the
molecular formula.
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With hydrazine, if we knew ONLY the empirical
formula to be NH2, and its molecular weight is
32.05 amu/molecule, we can figure out its
molecular formula:
1) Calculate EW from its empirical formula:
14.01 + 2(1.008) = 16.03 u/f.u.
2)  MW 
 32.05 u/molecule 

  
  1.999 (units  ?)
 EW 
 16.03 u/f.u. 
3) Multiple is 2 (must be very close to a whole
number).
Molecular formula = 2x(empirical formula)
2x(NH2)
ANS. Molecular formula is N2H4
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Molecular Formula
Example 3.12 p.128
Styrene, the raw material for
polystyrene foam plastics, has an
empirical formula of CH. Its
molecular mass is 104 g mol-1. What
is its molecular formula.
Do Practice Exercises 24 & 25 p.128.
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