Molecular Formula

  1. The molecular formula of a substance is the chemical formula that gives the actual number of atoms of each element in the substance.
  2. A molecular formula is the same as or a multiple of the empirical formula. 
  3. For example, the empirical of carbon dioxide is CO2 and the molecular formula is also CO2.
  4. Whereas, the empirical formula of ethane is CH3 while the molecular formula of ethane is C2H6.

Finding Molecular Formula 

Example
Given that the empirical formula of benzene is CH and its relative molecular mass is 78. Find the molecular formula of benzene. [Relative Atomic Mass: Carbon: 12; Hydrogen: 1]

Answer:
Let's say the molecular formula of benzene is CnHn.

The relative molecular mass of CnHn

= n(12) + n(1)
= 13n

13n = 78
n = 78/13 = 6

Therefore, the molecular formula of benzene
C6H6