A molecular formula is one in which all of the atoms in the compound are listed. For example, if I have the molecular formula ##C_2H_6##, I know that there are two carbon atoms and six hydrogen atoms in the molecule.

An empirical formula is one in which the ratios of the atoms of each element are listed. For the example above, there is a 1:3 ratio of carbon to hydrogen, so the empirical formula of this compound would be ##CH_3##.

In the case of ##P_4O_6##, the ratio of phosphorus to oxygen is 2:3, so the empirical formula is ##P_2O_3##.

OK.