Empirical formulae

The empirical formula gives the simplest ratio of the atoms within a compound. Some examples are given in the table below.

Name	Molecular formula	Empirical formula
methane	CH₄	CH₄
cyclohexene	C₆H₁₀	C₃H₅
benzene	C₆H₆	CH
ethanol	CH₃CH₂OH	C₂H₆O
ethanoic acid	CH₃COOH	CH₂O
propanal	CH₃CH₂CHO	C₃H₆O
butanoic acid	CH₃CH₂CH₂COOH	C₂H₄O
ethyl ethanoate	CH₃CH₂OOCCH₃	C₂H₄O

Points to note from this table:

If the ratio in the formula cannot be simplified, the empirical formula of the compound is the same as the molecular formula.
Different compounds can share the same empirical formula.
The molecular formula can always be worked out by multiplying the empirical formula by a whole number.

Empirical formula calculation

Combustion data can be used for these types of problems. A known mass of the unknown compound is burned in excess oxygen and the masses of carbon dioxide and water produced are measured. Work through the following example.

Example

A 6.00 g sample of an organic compound that was known to contain carbon, hydrogen and oxygen was burned in excess oxygen. This produced 13.64 g of carbon dioxide and 5.58 g of water. Calculate the empirical formula of the compound.

Click on the headings below to reveal the stages in the calculation.

Calculate the mass of carbon and nitrogen in sample

Calculate the mass of oxygen in sample

Calculate the empirical formula using the molar masses

The calculation should be set out in a table as seen here.

molar mass (M)	(12.01)	(1.008)	(16.00)
elements	C	H	O
mass (m)	3.72	0.624	1.66

ratio of moles	0.310	0.619	0.103
divide by smallest value	= 3.0	= 6.0	= 1.0
empirical formula	C₃H₆O

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The empirical formula is therefore C₃H₆O.

In this example it is known that the molecular formula of the substance is the same as the empirical formula.

In your notes name and draw the structures of two possible identities of the compound.

Click here to reveal possible answers

Describe a chemical test that could be used to confirm which of these two possibilities is the actual identity of the compound.

Hint: Think about the chemical properties of aldehydes and ketones.

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The above example shows you how to use chemical knowledge combined with knowledge of the formula to work out the complete structure of the unknown compound. There are more examples like this for you to complete later.