Empirical & Molecular Formulae (Edexcel IGCSE Chemistry)

Empirical & Molecular Formulae

• The molecular formula is the formula that shows the number and type of each atom in a molecule

  • E.g. the molecular formula of ethanoic acid is C₂H₄O₂

• 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

  • E.g. the empirical formula of ethanoic acid is CH₂O

• Organic molecules often have different empirical and molecular formulae

• The formula of an ionic compound is always an empirical formula

Calculating empirical and molecular formula

How to calculate empirical formulae

• Empirical formula calculations are very methodical

• Use a table and the following steps to complete an empirical formula calculation:

  1. Write the element

  2. Write the value given for each element

     • This may be given as a mass, in g, or as a percentage

     • There are exam questions where you are required to calculate the value of one of the elements

  3. Write the relative atomic mass of each element

  4. Calculate the moles of each element

     • Moles = 

  5. Calculate the ratio of elements

     • Divide all the moles by the smallest number of moles

     • If you get a ratio that does not have whole numbers, you multiply by an appropriate number to make all the values into whole numbers

  6. Write the final empirical formula

How to calculate molecular formula

• Molecular formula gives the actual numbers of atoms of each element present in the formula of the compound

Table showing the relationship between empirical and molecular formulae

• To calculate the molecular formula:

  1. Find the relative formula mass of the empirical formula

     • Add the relative atomic masses of all the atoms in the empirical formula

  2. Use the following equation:

     • 

  3. Multiply the number of each element present in the empirical formula by the number from step 2 to find the molecular formula

Deducing formulae of hydrated salts

• A hydrated salt is a crystallised salt that contains water molecules as part of its structure

• The formula of a hydrated salt shows the water molecules, e.g. CuSO₄•2H₂O

  • The • symbol shows that the water present is water of crystallisation

• The formula of hydrated salts can be determined experimentally by:

  • Weighing a sample of the hydrated salt

  • Heating it until the water of crystallisation has been driven off

    • This is achieved by heating until a constant mass

  • Re-weighing the anhydrous salt

• From the results, you can determine the mass of anhydrous salt and the mass of the water of crystallisation

• Applying a similar approach to deducing empirical formulae, the formula of the hydrated salt can be calculated

How to calculate water of crystallisation

• The steps for empirical formula can be adapted for hydrated salt / water of crystallisation calculations 

  • Instead of writing elements, write the two components of a hydrated salt

    • The salt

    • Water

  • Instead of writing relative atomic mass, write the relative molecular / formula mass of the salt and water

• Use a table and the following steps to complete the calculation:

  1. Write the salt and water

  2. Write the value given for the salt and water 

     • There are exam questions where you are required to calculate one of these values

  3. Write the relative molecular / formula mass of the salt and water

  4. Calculate the moles of the salt and water

     • Moles = 

  5. Calculate the ratio salt : water 

     • Divide all the moles by the smallest number of moles

     • The calculation should give a ratio of 1 salt : x water

  6. Write the final hydrated salt formula