So we have molecular formulae and empirical formulae. How do they differ?
A molecular formula shows the exact number of atoms of each element present in a substance. For example:

Ethane has the formula C₂H₆. This formula shows that each molecule of ethane has two atoms of carbon and 6 atoms of hydrogen.

The empirical formula shows the simplest ratio of atoms or ions present in the substance. For example:

Ethane has the empirical formula CH₃.

Some substances have the same empirical formula as their molecular formula. Carbon dioxide (CO₂) and methane (CH₄) are two such substances. The molecular formula for these substances can not be simplified any further.