Finding the
Empirical Formula
of a Compound
© Chemworks
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The Empirical Formula is just the simplest
formula of a compound.
For example the empirical formula
for water is H2O
It shows that hydrogen and oxygen
are in the ratio of 2 : 1
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However not all chemicals have their formulae
in such a simple ratio.
For example the chemical formula
for hydrogen peroxide is H2O2
It shows that hydrogen and oxygen
are in the ratio of 2 : 2
The ratio could be simplified to 1 : 1
So the empirical formula is HO, not H2O2
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The same thing happens with
dinitrogen tetroxide.
The chemical formula
for dinitrogen tetroxide is N2O4
It shows that the nitrogen and oxygen
are in the ratio of 2 : 4
The ratio could be simplified to 1 : 2
So the empirical formula is NO2, not N2O4
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Example 1: A compound contains 24g carbon and
8g hydrogen. What is its empirical formula?
Elements
C
H
Mass
24g
8g
g.f.mass
12
1
moles
24
12
8
1
ratio
2
8
simplest ratio
1
4
So the empirical formula is CH4
Let’s try another example …
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Example 2: A compound is 27.3% carbon and
72.7% oxygen. What is its empirical formula?
Elements
Mass
C
27.3g
O
72.7g
We seem to have a problem as there are no
masses, only percentages.
But if we assume that we had 100g of the
compound it would contain 27.3g of carbon and
72.7g of oxygen.
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Example 2: A compound is 27.3% carbon and
72.7% oxygen. What is its empirical formula?
Elements
Mass
C
O
27.3g
72.7g
g.f.mass
12
16
moles
27.3g
12
72.7g
16
ratio
2.275
4.544
ratio
2.275
2.275
4.544
2.275
This does not look like a simple ratio, but let’s
divide
both
numbers
by
the
smallest
number.
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Example 2: A compound is 27.3% carbon and
72.7% oxygen. What is its empirical formula?
Elements
Mass
C
O
27.3g
72.7g
g.f.mass
12
16
moles
27.3g
12
72.7g
16
ratio
2.275
4.544
ratio
2.275
2.275
4.544
2.275
ratio
1
2
So the formula for the compound is CO2
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Now try the following questions:
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Calculate the empirical formula for:
a compound that is 40% sulphur and 60% oxygen
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Elements
S
O
Mass
40g
60g
g.f.mass
32
16
moles
40
32
60
16
ratio
1.25
3.75
ratio
1.25
1.25
3.75
1.25
ratio
1
3
The empirical formula is SO3
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Calculate the empirical formula for:
a compound that is 80% copper and 20% oxygen
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Elements
Cu
O
Mass
80g
20g
g.f.mass
64.5
16
moles
80
64.5
20
16
ratio
1.24
1.25
ratio
1.24
1.24
1.25
1.24
ratio
1
1
The empirical formula is CuO
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Calculate the empirical formula for a compound
that is 2% hydrogen, 32.7% sulphur
and 65.3% oxygen
Elements
H
S
O
Mass
2g
32.7g
g.f.mass
1
32
16
moles
2
1
32.7
32
65.3
16
ratio
2
1.02
4.08
ratio
2
1
4
65.3%
The empirical formula is H2 SO4
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Calculate the empirical formula for a compound
that is 43.4% sodium, 11.3% carbon
and 45.3% oxygen
Elements
Na
Mass
43.4g
11.3g
45.3%
23
12
16
moles
43.4
23
11.3
12
45.3
16
ratio
moles
1.89
1.89
0.94
0.94
0.94
0.94
2.83
2.83
0.94
ratio
2
1
3
g.f.mass
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C
O
The
empirical
formula is Na2 CO3
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16g of a copper oxide compound is reduced to
12.8g copper. What is the empirical formula of
the copper oxide compound?
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Elements
Cu
O
Mass
12.8g
3.2g
g.f.mass
64.5
32
moles
12.8
64.5
3.2
32
ratio
moles
0.2
0.2
0.1
0.1
0.1
0.1
ratio
2
1
The empirical
formula is Cu2O
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Well done!
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