Empirical & Molecular Formulae (CIE AS Chemistry)

• The molecular formula is the formula that shows the number and type of each atom in a molecule
  • Eg. the molecular formula of ethanoic acid is C₂H₄O₂

• The empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
  • Eg. the empirical formula of ethanoic acid is CH₂O

• Organic molecules often have different empirical and molecular formulae
• Simple inorganic molecules however have often similar empirical and molecular formulae
• Ionic compounds always have similar empirical and molecular formulae

Empirical formula

• Empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
• It is calculated from knowledge of the ratio of masses of each element in the compound
• The empirical formula can be found by determining the mass of each element present in a sample of the compound
• It can also be deduced from data that give the percentage compositions by mass of the elements in a compound

Worked Example: Empirical formula from mass

Worked Example 2: Empirical formula from %

Molecular formula

• The molecular formula gives the exact numbers of atoms of each element present in the formula of the compound
• The molecular formula can be found by dividing 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

Worked example: Calculating molecular formula

 Answer

• Step 1: Calculate relative formula mass of empirical formula

Relative formula mass = (C x 4) + (H x 10) + (S x 1)

Relative formula mass = (12 x 4) + (1 x 10) + (32 x 1)

Relative formula mass = 90

• Step 2: Divide relative formula mass of X by relative formula mass of empirical formula

Ratio between M_r of X and the M_r of the empirical formula = 180/90

Ratio between M_r of X and the M_r of the empirical formula = 2

• Step 3: Multiply each number of elements by 2

(C_{4 x 2}) + (H_{10 x 2}) + (S_{1 x 2})     =    (C₈) + (H₂₀) + (S₂)

Molecular Formula of X is C₈H₂₀S₂