A molecular formula indicates, with subscripts, the number of atoms of each element in one molecule of a compound. In comparison, the empirical formula gives the simplest whole number ratio of atoms in a molecule. As an example, the molecular formula for glucose is `C_6H_12O_6` . This means that each molecuse of glucose is made up of six carbon atoms, twelve oxygen atoms and six hydrogen atoms. The empirical formula for glucose is `CH_2O` . This shows that the ratio of carbon to hydrogen to oxygen is 1:2:1, but doesn't provide any more information about the molecule.
The molecular formula is a whole-number multiple of the empiricial formula. It's possible for that whole number to be one and the two formulas to be the same, for example methane has the molecular formula `CH_4` which is also the simplest or empirical formula.
The empirical formula can be determined from the percent composition or mass composition of a compound, and the molecular formula then be determined if the empI rival formula is known. For example, if you just knew the empirical formula for glucose and that it's molar mass is 180 g/mol, you would divide 180 by the molar mass of the empirical formula, 30. This gives the whole number 6, so the molecular formula is 6(`CH_2O` ) or `C_6H_12O_6` .