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Empirical Formula
Recall: Chemical Formula – Relative
number of various types of atoms present in
a compound
Suppose a compound ‘A’ contains C, H, and
O. We analyze 0.2015g of compound ‘A’
and find it contains 0.0806g of C, 0.01353g
of H and 0.1074g of O. How can we
determine its chemical formula?
MASS → MOLES!
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 1 mol C 
C = 0.0806 × 
 = 0.00671 mol C
12.01
g
C


 1 mol H
H = 0.01353 g × 
 1.008 g H

 = 0.01342 mol H

 1 mol O 
O = 0.1074 g O × 
 = 0.00671 mol O
 16.00 g O 
What do these results tell us about the chemical
formula?
1. The compound contains the same number of
C and O atoms
2. There are twice as many H atoms as C atoms
and O atoms
The relative number of C,H, and O atoms is
expressed by the formula CH2O
The formula that expresses the smallest whole
number ratio of atoms present in a compound is
called the empirical formula (simplest formula).
The actual formula is called the molecular formula.
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How to calculate the empirical formula from the
mass
An oxide of aluminum is formed by the reaction of
4.151g of aluminum with 3.692g of oxygen. What
is the empirical formula?
1. Determine the mass of each element
2. Determine the number of moles of each type
of atom
 1 mol Al 
4.151g Al × 
 = 0.1539 mol Al atoms
26.98
g
Al


 1 mol O 
3.692 g O × 
 = 0.2308 mol O atoms
16.00
g
O


Because chemical formulas use whole numbers,
next find the integer ratio of atoms
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3. Divide the number of moles of each element by
the smallest number of moles
 0.1539 mol Al 

 = 1.000 mol Al atoms
0.1539


 0.2308 mol O 

 = 1.500 mol O atoms
0.1539


We now know the compound contains 1.500 moles
of O and 1.000 moles of Al. We could leave it like
this but atoms are whole in compounds! So….
4. If the numbers are not integers, multiply by the
smallest integer that will convert all of the numbers
to whole numbers. These numbers are the
empirical formula
1.500 mol O × 2 = 3 mol O atoms
1.000 mol Al × 2 = 2 mol Al atoms
The empirical formula = Al2O3
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Determining the Molecular Formula
• Obtain the empirical formula
• Calculate the mass corresponding to the
empirical formula
• Calculate the ratio
molar mass
empirical formula mass
• The integer from the previous step represents
the number of empirical formula units in one
molecule. When the empirical formula
subscripts are multiplied by this integer, the
molecular formula results. This procedure is
summarized by the equation:


molar mass
molecular formula = ( empirical formula ) × 

 empirical formula mass 
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