Empirical and Molecular Formula - Practice Questions & MCQ

Chemical Formula: 
A chemical formula represents the combination of atoms of all the elements which make up a compound. It represents the relative ratio of atoms of its constituent elements. In case of a compound, it represents one molecule, one mole, one gram molecular weight of the compound.
Example, CuSO₄.5H₂O represent one molecule, one mole and one gram molecular weight of hydrated copper sulphate.

Empirical Formula: 
It is the simplest ratio of the number of atoms of different elements present in one molecule of a compound. It does not represent the actual number of atoms of different elements present in one molecule of the compound.

Step 1. Conversion of mass percent to grams.

Step 2. Convert into number moles of each element.

Step 3. Divide the mole value obtained above by the smallest number.

Step 4. Write the empirical formula by mentioning the numbers after writing the symbols of respective elements.

Step 5. Writing molecular formula (Molecular formula) 

Molecular Formula: 
It shows the actual number of atoms of different elements present in one molecule of a compound.

• n = (Molecular weight) / (Empirical formula weight)

• Molecular Formula = Empirical formula X n

• Molecular formula of a substance can be determined by methods like Vapour Density method, Elevation in boiling point method, Depression in freezing point methods etc.

• The molecular weight of volatile compounds is determined by Victor Meyer’s Method.

• Molecular weight = 2 x Vapour Density

• For some compounds the molecular formula and empirical formula may be same also.