1. Reading Guide 10.3b Empirical Formulas Molecular Formulas
Chemistry

2. Empirical Formula
The formulas for some compounds show a basic ratio of elements. Multiplying that ratio by any factor can make formulas for other compounds.
The basic ratio is called empirical formula and the percent composition helps calculate the empirical formula.
For example the empirical formula may be CO2. This shows the kinds and lowest relative count of atoms or moles of atoms in a compound.

4. An empirical formula may or may not be the same as a molecular formula
An empirical formula may or may not be the same as a molecular formula. The lowest ratio of hydrogen to oxygen in hydrogen peroxide is 1:1, so the empirical formula is HO, but the molecular formula has twice the number of atoms as the empirical formula. The molecular formula is H2O2.
The empirical formula of a compound shows the smallest whole-number ratio of the atoms in the compound.

6. The molecular formula tells the actual number of each kind of atom present in a molecule of the compound.
For carbon dioxide the empirical and molecular formulas are the same - CO2.

7. Molecular Formulas
The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula.
Once you have figured out the empirical formula of a compound you can determine its molecular formula, but you have to know the compounds molar mass.
A chemist will use a mass spectrometer to figure out the molar mass.

8. From the empirical formula you can figure out the empirical formula mass. You can divide the experimentally determined mass by the empirical formula mass. This gives the number of empirical formula units in a molecule of the compound and it is a multiplier to convert the empirical formula to the molecular formula.

10. An example for hydrogen peroxide:
The empirical formula for hydrogen peroxide is HO. Its empirical formula mass is 17.0 g/mol. The molar mass of H2O2 is 34.0 g/mol.
34.0g/mol / 17.0 g/mol = 2
So to find the molecular formula of hydrogen peroxide multiply the subscripts of the empirical formula by 2.