Empirical & Molecular Formula

Calculating Empirical & Molecular Formulaevideo

Calculating Empirical & Molecular Formulae

Calculating Empirical Formula

• 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 Molecular Formula

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

• To calculate the molecular formula:

  • Step 1: Find the relative formula mass of the empirical formula

  • Step 2: Use the following equation:

$\frac{\mathrm{relative} \mathrm{formula} \mathrm{mass} \mathrm{of} \mathrm{molecular} \mathrm{formula}}{\mathrm{relative} \mathrm{formula} \mathrm{mass} \mathrm{of} \mathrm{empirical} \mathrm{formula}}$

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

Table showing the Relationship between Empirical and Molecular Formula

Deducing formulae of hydrated salts

• 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, then reweighing the now 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