Empirical Formula

Step by step example showing how to work out the empirical formula

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  • Created on: 27-05-10 15:26
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Empirical Formula
The empirical formula of a compound is the simplest form of the ratio of the atoms of
different elements in it. The molecular formula tells the actual number of each kind of atom
in a molecule of the substance.
Example (1)
An 18.3g sample of a hydrated component contains 4g of calcium, 7.1g of chlorine and 7.2g
of water only.
Ca Cl H20
Mass (g) 4.0 7.1 7.2
Molar Mass 40 35.5 18
Number of moles
Mass 0.1 0.2 0.4
Molar Mass
Moles 0.1 ÷ 0.1 0.2 ÷ 0.1 0.4 ÷ 0.1
Smallest Mole
Simplest ratio of 1 2 4
relative amount
Example (2)
An organic compound was analysed and was found to have the following percentage
composition by mass 48.8% carbon, 13.5%hydrogen and 37.7% nitrogen. Calculate the
empirical formula of the compound.
C H N
Mass 48.8 13.5 37.7
Molar Mass 12 1 14
Number of moles 4.0666 13.5 2.6928
Mass
Molar Mass
Moles 13.5 ÷ 2.6928 =5 2.6928 ÷ 2.6928 =
Smallest Mole 4.0666 ÷ 2.6928 = 1.5 1
NOTE : To make an
integer multiply by 2
2 10 2
Simplest ratio of 2 10 2
relative amount

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Example (3)
Ammonium carbonate contains 15.38% of carbon, 7.69% of hydrogen, 35.9 % of nitrogen
and 41.03% of oxygen by mass.Use the data to confirm that the empirical formula of
ammonium carbonate is CH6N2O2.
C H N O
Mass (g)
15.38 7.69 35.9 41.03
Molar Mass
12 1 14 16
Number of moles
Mass 1.28166 7.69 2.564 2.564
Molar Mass
Moles 1.28166 ÷ 1.28166 7.69 ÷ 1.28166 2.564 ÷ 1.28166 2.564 ÷ 1.…read more

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