Empirical Formula
Contents
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. 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. 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. | State the mass of all reactants. | State the mass of all reactants.
|
Find the number of moles of each element.
Number of Moles = (Mass/Relative Atomic Mass) Relative Atomic Mass of Oxygen = 16g
|
Find the number of moles of each element.
Number of Moles = (Mass/Relative Atomic Mass) Relative Atomic Mass of Carbon = 12g
|
Find the number of moles of each element.
Number of Moles = (Mass/Relative Atomic Mass) Relative Atomic Mass of Sulphur = 32g
|
Find the Ratio:
8:4 2:1 |
Find the Ratio:
5:10 1:2 |
Find the Ratio:
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
Edexcel
- Empirical formula, page 217, GCSE Combined Science, Pearson Edexcel
- Empirical formulae, page 73, GCSE Chemistry, Pearson, Edexcel
- Empirical formulae, pages 71, 72, 77, 78, GCSE Chemistry, CGP, Edexcel
- Empirical formulas, pages 27, 30, GCSE Chemistry; The Revision Guide, CGP, Edexcel
- Empirical formulas, pages 90, 93, GCSE Combined Science; The Revision Guide, CGP, Edexcel