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Empirical formulae

• Work out the formulae of the following compounds:
• Sodium sulphide Na2S
• Calcium chloride

CaCl

• Aluminium oxide Al3O

Empirical formulae

• Work out the formulae of the following compounds:
• Sodium sulphide
• Na 2 S
• Calcium chloride
• CaCl (^2)
• Aluminium oxide
• Al 2 O 3

Working out empirical formulae 1

• If you know the molecular formula, working out the empirical formula is straightforward.
• For example, the molecular formula of ethane is C 2 H 6
• The carbon and hydrogen atoms are in the ratio 2:
• This ratio, in its lowest terms, is 1:
• Therefore the empirical formula of ethane is CH 3.

Working out empirical formulae 1

• Work out the empirical formulae of the following:
• C 6 H 12 O 6

• C 2 H 4

• C 6 H 3 N 3 O 6

Working out empirical formulae 2

• The empirical formula can be worked out by using data about % composition or the mass of each element in the compound.
• For example:
• Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen.

Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen.

• Write down the symbol for each element

Symbol K O % comp or mass 83 17 A (^) r n = divide by smallest Lowest ratio

Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen.

• Write down the symbol for each element
• Write down the mass or % composition of each element
• Divide by the relative atomic mass (A (^) r ) of each, to find the number of moles.

Symbol K O % comp or mass 83 17 A (^) r 39.1 16 n = 2.12 1. divide by smallest Lowest ratio

Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen.

• Write down the symbol for each element
• Write down the mass or % composition of each element
• Divide by the relative atomic mass (A (^) r ) of each, to find the number of moles.
• Divide each of the values by the smallest number of moles of the elements present.

Symbol K O % comp or mass 83 17 A (^) r 39.1 16 n = 2.12 1. divide by smallest 1.06 1. Lowest ratio

Practice Questions

1. What is the empirical formula of a compound containing 51.3% calcium and 48.7% fluorine?
2. What is the empirical formula of a compound containing 26.45g of aluminium and 23.55g of oxygen?

Practice Questions

1. What is the empirical formula of a compound containing 51.3% calcium and 48.7% fluorine?

• CaF (^2)

Converting empirical formulae to

molecular formulae

• What is the empirical formula of a hydrocarbon containing 85.7% carbon?
• The same hydrocarbon has a molar mass of
  42. What is its molecular formula?

Converting empirical formulae to

molecular formulae

• A hydrocarbon contains carbon and oxygen only, hence the remaining 14.3% must be hydrogen.
• The empirical formula is CH (^2)

Symbol C H % comp or mass 85.7 14. A (^) r 12 1 n = 7.14 14. divide by smallest 7.14 7. Lowest ratio 1 2