Empirical Formula

Key Stage 4

Meaning

An empirical formula is the simplest ratio of the different types of atom in a compound.

About Empirical Formulae

The empirical formula of a compound may not be the same as the chemical formula:

• Ethane - Chemical Formula C₂H₆, Empirical Formula CH₃
• Ethene - Chemical Formula C₂H₄, Empirical Formula CH₂
• Propene - Chemical Formula C₃H₆, Empirical Formula CH₂
• Glucose - Chemical Formula C₆H₁₂O₆, Empirical Formula CH₂O
• Lactic Acid - Chemical Formula C₃H₆O₃, Empirical Formula CH₂O

Empirical formulae are calculated from the amount of atoms in a chemical reaction.

The number of atoms can be found if you know the mass of different elements and the relative atomic mass of the elements in the reaction.

Finding the Empirical Formula

Number of Atoms	Skeletal Diagram	Ball and Stick Model
100 atoms of Hydrogen react completely with 50 atoms of Oxygen.
The ratio of Hydrogen atoms to Oxygen atoms H:O 100:50 2:1 So the empirical formula is H2O	In this diagram there are 4 Carbon atoms, 10 Hydrogen atoms. The ratio of atoms is: C:H 4:10 2:5 So the empirical formula is C2H5	In this diagram there are 4 Carbon atoms, 10 Hydrogen atoms and 2 Oxygen atoms. The ratio of atoms is: C:H:O 4:10:2 2:5:1 So the empirical formula is C2H5O

64g of Oxygen is found to react completely with 8g of Hydrogen. Find the empirical formula for the product.	60g of Carbon is found to react completely with 160g of Oxygen. Find the empirical formula for the product.	416g of Sulphur is found to react completely with Oxygen to produce 832g of product. Find the empirical formula for the product.
State the mass of all reactants. Mass of Oxygen = 64g Mass of Hydrogen = 8g	State the mass of all reactants. Mass of Carbon = 60g Mass of Oxygen = 416g	State the mass of all reactants. Mass of Sulphur = 416g Mass of Oxygen = 416g
Find the number of moles of each element. Number of Moles = (Mass/Relative Atomic Mass) Relative Atomic Mass of Oxygen = 16g Moles of Oxygen = 64/16 Moles of Oxygen = 4 mole Relative Atomic Mass of Hydrogen = 1g Moles of Hydrogen = 8/1 Moles of Hydrogen = 8 mole	Find the number of moles of each element. Number of Moles = (Mass/Relative Atomic Mass) Relative Atomic Mass of Carbon = 12g Moles of Carbon = 60/12 Moles of Carbon = 5 mole Mass of Oxygen = 160g Relative Atomic Mass of Oxygen = 16g Moles of Oxygen = 160/16 Moles of Oxygen = 10 mole	Find the number of moles of each element. Number of Moles = (Mass/Relative Atomic Mass) Relative Atomic Mass of Sulphur = 32g Moles of Sulphur = 416/32 Moles of Sulphur = 32 mole Relative Atomic Mass of Oxygen = 16g Moles of Oxygen = 416/16 Moles of Oxygen = 26 mole
Find the Ratio: Hydrogen:Oxygen 8:4 2:1	Find the Ratio: Carbon:Oxygen 5:10 1:2	Find the Ratio: Sulphur:Oxygen 13:26 1:2
The empirical formula of the product is H2O	The empirical formula of the product is CO2	The empirical formula of the product is SO2

References

AQA

Formula, formulae; empirical, pages 62-3, 90-1, 252, GCSE Chemistry; Student Book, Collins, AQA