Edexcel IGCSE Chemistry

Revision Notes

1.5.8 Empirical & Molecular Formulae

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

Calculate Empirical & Molecular Formulae

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

Example:

A compound that contains 10 g of hydrogen and 80 g of oxygen has an empirical formula of H2O. This can be shown by the following calculations:

Amount of hydrogen atoms = mass in grams ÷ Ar of hydrogen = (10 ÷ 1) = 10 moles

Amount of oxygen atoms = mass in grams ÷ Ar of oxygen = (80 ÷ 16) = 5 moles

The Ratio of Moles of Hydrogen Atoms to Moles of Oxygen Atoms

The Ratio of Moles of Hydrogen Atoms to Moles of Oxygen Atoms table, IGCSE & GCSE Chemistry revision notes

 

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

Molecular formula: gives the exact numbers of atoms of each element present in the formula of the compound

  • Divide the relative formula mass of the molecular formula by the relative formula mass of the empirical formula
  • Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Relationship between Empirical and Molecular Formula

Relationship between Empirical _ Molecular Formula table, IGCSE & GCSE Chemistry revision notes

Worked Example

The empirical formula of X is C4H10S1 and the relative formula mass of X is 180. What is the molecular formula of X?

Relative atomic masses:       carbon : 12      hydrogen : 1      sulfur : 32

Step 1 – Calculate the relative formula mass of the empirical formula

(C x 4) + (H x 10) + (S x 1)    =   (12 x 4) + (1 x 10) + (32 x 1)   =   90

Step 2 – Divide the relative formula mass of X by the mass of the empirical formula

180 / 90 = 2

Step 3 – Multiply each number of elements by 2

(C4 x 2) + (H10 x 2) + (S1 x 2)     =    (C8) + (H20) + (S2)

Molecular Formula of X = C8H20S2

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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