1.2.2 Empirical & Molecular Formula

• 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 atoms of each element present in one molecule or formula unit of a compound
  • E.g. the empirical formula of ethanoic acid is CH₂O

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 gives the percentage compositions by mass of the elements in a compound

• The above example shows how to calculate empirical formula from the mass of each element present in the compound
• The example below shows how to calculate the empirical formula from percentage composition

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

Answer

Step 1: Calculate relative mass of the empirical formula

• • Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
  • Relative empirical mass = (12 x 4) + (1 x 10) + (32 x 1)
  • Relative empirical mass = 90

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

• • 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

• • (C₄ x 2) + (H₁₀ x 2) + (S x 2)     =    (C₈) + (H₂₀) + (S₂)
  • Molecular Formula of X is C₈H₂₀S₂