Percentage composition: It is the percentage of each elements present in 100g of a substance.
Percentage composition (mass percent) of an element
= Mass of that element in the compound x 100                
           Molar mass of the compound


Empirical Formula: It represents the simplest whole number ratio of various atoms present in a compound. e.g., CH is the empirical formula of benzene (C6H6).

Molecular Formula: It shows the exact number of different of atoms present in a molecule of a compound. e.g., C6H6 is the molecular formula of benzene.

Relationship between empirical and molecular formulae:
Molecular formula = n × Empirical formula
  Where; n = Molecular Mass
                     Empirical Formula Mass


Following Steps Should be Followed to Determine Empirical Formula of the Compound:
Step 1: Conversion of mass per cent of various elements into grams.
Step 2: Convert mass obtained into number of moles.
Step 3: Divide the mole value obtained by the smallest mole value (out of the mole value of various elements calculated)
Step 4: If the ratios are not whole numbers, then they may be converted into whole number by multiplying with the suitable coefficient.
Step 5: Write empirical formula by mentioning the numbers after writing the symbols of respective elements.
For Example:
Que. A compound on analysis gave the following percentage composition Na=14.31% S=9.97% H=6.22% O=69.5%. Calculate the molecular formula of the compound on the assumption that all hydrogen in the compound present in the combination with oxygen as water of crystallisation. Molecular mass of the compound is 322. Na=23 S=32 O=16 H=1.
Ans.


Empirical Formula = Na2SH20O20

We Know 
Molecular formula = n × Empirical formula
Where; n = Molecular Mass
                   Empirical Formula Mass

 n = 322 = 1
       322
Molar Formula = 1 × Na2SH20O20 = Na2SH20O20