An empirical formula displays the simplest whole number ratios of the number of atoms for the different elements in a chemical formula. A molecular formula ( which applies to molecules, covalent compounds) displays the actual number of atoms in a given molecule.
Consider the molecule, C2H6, which is ethane. This molecule consists of 2 molecules of carbon to six molecules of hydrogen.
Therefore, this formula is the molecular formula.
The ratio of carbon atoms to hydrogen atoms is 2 to 6, or 1 to 3 if it is reduced. The reduced form of the formula is C1H3 or simply CH3.
The name empirical derives from something that is obtained through experimentation. So, empirical formulas are determined through experimentation.
Here is a video which discusses this topic.
video from: Noel Pauller