The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound 

• The empirical formula of an organic molecule is often different to its molecular / chemical formula

  • For example, ethanoic acid has the chemical formula CH3COOH or C2H4O2 but its empirical formula is CH2O

• The molecular / chemical formula of an ionic compound is always its empirical formula 

  • For example, sodium chloride has the chemical formula NaCl, which is also its empirical formula

Deducing formulae of ionic compounds

• Simple molecules are formed when non-metals react together to form compounds

• Ionic compounds involve the metal losing electrons and the non-metal gaining electrons to form ions

• Some ions that you will be expected to be able to use, because they are stated in the exam specification, include:

  • Hydrogen ions, H+ - sometimes referred to as protons

  • Group 1 ions, e.g. Li+, Na+, K+ 

  • Group 7 ions, F–, Cl–, Br– 

  • Copper(II) ions, Cu2+ 

  • Iron(II) ions, Fe2+ 

  • Iron(III) ions, Fe3+ 

• There are some polyatomic (containing more than one atom) ions stated in the exam specification: 

  • Carbonate ions, CO32– 

  • Sulfate ions, SO42– 

  • Hydroxide ions, OH– 

  • Nitrate ions, NO3–

How to determine the formulae of ionic compounds

Ionic compounds typically have no overall charge

• This means that the size of any positively charged ion is cancelled by the size of any negatively charged ion

• Careful: This should not be confused with an atom having no overall charge

Direct comparison

• The formula of an ionic compound can be determined by directly comparing the charges of the ions:

  • For example, iron(II) sulfate

    • The iron(II) ion is Fe2+, which means that it has a 2+ or +2 charge

    • The sulfate ion is SO42–, which means that it has a 2– or –2 charge

    • The charges cancel each other out

      • Mathematically, (+2) + (–2) = 0

    • This means that one SO42– ion is needed to cancel the +2 charge on Fe2+ 

    • Therefore, the formula of iron(II) sulfate is FeSO4 

The swap-and-drop method

When the ions in the ionic compound have different charges, it can be easier to use the swap-and-drop method

• Careful: If you use this method with ions that have the same charge, then you must give the simplest whole number ratio to get the correct answer

• For example, copper(II) chloride:

  • The copper(II) ion is Cu2+, which means that it has a 2+ or +2 charge

  • The chloride ion is Cl–, which means that it has a 1– or –1 charge

  • The size of the charge on the copper(II) ion indicates the number of chloride ions needed, and the size of the charge on the chloride ion indicates the number of copper(II) ions needed