2.3  Chemical Formula

• The chemical formula represents a chemical compound.
• Below are some of the examples:

 

Name of compound	Chemical formula	Number of each element in the compound
Helium	He	One helium atom
Oxygen	\(O_2\)	2 oxygen atoms
Water	\(H_2O\)	2 hydrogen atoms and 1 oxygen atom
Ammonia	\(NH_3\)	1 nitrogen atom and 3 hydrogen atoms
Sulphuric acid	\(H_2SO_4\)	2 hydrogen atoms, 1 sulphur atom and 4 oxygen atoms

 

 Example of the formation of chemical formula

• The chemical formula of iron(III) chloride \(FeCl_3\)
  • Iron(III) ion = \(Fe^{+3}\)
  • Chlorine ion = \(Cl^{-}\)
  • Iron, Fe has three positive charges, and Cl has one negative charge.
  • When these two types of atoms are reacted together, the charge from both atoms needs to be balanced.
  • 3 positive charge from iron needs three negative charges to make the charge equal to zero.
  • Since chlorine only has on negative charge; thus, the iron required three chlorine atom to neutralise the three positive charge.
  • Therefore the chemical formula would become, \(FeCl_3\).  

 

Formation of iron(III) chloride compound

 

 

Empirical formula

• There are several steps in determining the empirical formula of a compound. 

 

Example

Element C H Mass (g) \(96\) \(18\) Number of mole \(\dfrac{96}{12}=8\) \(\dfrac{18}{1}=18\) Mole ratio \(8\) \(18\) Simplest mol ratio \(\dfrac{8}{2} =4\) \(\dfrac{18}{2} = 9\) Empirical formula \(C_4H_9\)