Empirical Formula and Molecular Formula:

While the empirical formula is the simplest form of a compound,
the molecular formula is the form of the term as it would appear
in a chemical equation. The empirical formula and the molecular
formula can be the same, or the molecular formula can be any
positive integer multiple of the empirical formula.

Examples of empirical formulas:

AgBr, Na2S, C6H10O5.

Examples of molecular formulas:

P2, C2O4, C6H14S2, H2, C3H9.

One can calculate the empirical formula from the masses or
percentage composition of any compound.
We have already discussed percent composition in the section above.
If we only have mass, all we are doing is essentially eliminating
the step of converting from percentage to mass.

Example:

Calculate the empirical formula for a compound that has
43.7 g P (phosphorus) and 56.3 grams of oxygen.
First we convert to moles:





Next we divide the moles to try to get an even ratio:





When we divide, we did not get whole numbers so we must multiply by two (2).
The answer = P2O5

Calculating the molecular formula once we have the empirical formula is easy. If we know the empirical formula of a compound, all we need to do is divide the molecular mass of the compound by the mass of the empirical formula. It is also possible to do this with one of the elements in the formula; simply divide the mass of that element in one mole of compound by the mass of that element in the empirical formula. The result should always be a natural number.

Example:

If we know that the empirical formula of a compound is HCN and we are told that a 2.016 grams of hydrogen are necessary to make the compound, what is the molecular formula? In the empirical formula hydrogen weighs 1.008 grams. Dividing 2.016 by 1.008 we see that the amount of hydrogen needed is twice as much. Therefore the empirical formula needs to be increased by a factor of two (2).
The answer is: H2C2N2.