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