• 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 Mr?of X and the Mr?of the empirical formula = 180/90

Ratio between Mr?of X and the Mr?of the empirical formula = 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?is C₈H₂₀S₂