Molecular Formula

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]

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

Example:

What is the mass of metal X that can combine with 14.4g of oxygen to form X oxide with molecular formula X2O3. (RAM: O = 16; X = 56 )

Number of mole of oxygen
= 14.4/16
=0.9 mol

From the molecular formula, we learn that the ratio of element X to oxygen X:O = 2:3
Therefore, the number of mole of X =0.9× 2/3 =0.6 mol

Number of mole,
n =  mass/Molar mass
0.6 = mass/56*
mass = 33.6g

The mass of element X = 33.6g

*Molar mass of a substance = Relative atomic mass of the substance