Empirical Formulae (Edexcel GCSE Chemistry: Combined Science)

Empirical Formula from Reacting Masses

• An empirical formula gives the simplest whole number ratio of atoms of each element in the compound
• It is calculated from knowledge of the ratio of masses of each element in the compound
• Suppose a compound contains 10 g of hydrogen and 80 g of oxygen. We can calculate the empirical formula by

  1. Dividing the reacting masses by the relative atomic mass of each element (this gives the moles)
  2. Divide each result by the lowest number obtained to give the simplest ratio

• This can be shown by the following calculations:
  • Amount of hydrogen atoms = Mass in grams ÷ A_r of hydrogen = (10 ÷ 1) = 10 moles
  • Amount of oxygen atoms = Mass in grams ÷ A_r of oxygen = (80 ÷ 16) = 5 moles

• The ratio of moles of hydrogen atoms to moles of oxygen atoms:

• Since equal numbers of moles of atoms contain the same number of atoms, the ratio of hydrogen atoms to oxygen atoms is 2:1
• Hence the empirical formula is H₂O

Empirical Formula from Molecular Formula

• By inspection you simply reduce the molecular formula to the simplest ratio and you have the empirical formula
• Sometimes the empirical formula is the same as the molecular formula, as in the example of methane
• The formula of ionic compounds is always the empirical formula

Relationship between Empirical and Molecular Formula

Molecular Formula from Empirical Formula

• To find the molecular formula from the empirical formula you need to know the relative formula mass of the substance
• The steps involved are:
  1. Find the empirical formula mass
  2. Divide the relative formula mass by the empirical formula mass to obtain the multiple
  3. Multiple this number by the empirical formula to obtain the molecular formula