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.

How to find out the empirical formula and the molecular formula in case the % composition of the compound is given to us

Step 1. Conversion of a mass percent to grams.

Step 2. Convert grams into the number of 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 obtained above after writing the symbols of respective elements.

Step 5. Write molecular formula (Molecular formula) with the help of the information given. 

Molecular formula is a whole number multiple of the empirical formula

where n is whole number.

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 weight can be directly given or some other information like Vapour density can be given which will enable us to calculate the molecular weight.

• Molecular weight = 2 x Vapour Density

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