[chem] empirical formula and molecular formula

The molecular formula of a molecular compound represents the composition of a molecule of this compound, i.e. the types of atom and the number of each type of atom in the compound. Only compounds with molecular structure can be represented by molecular formulae.

The empirical formula of a compound represents the simplest ratio of the numbers of atoms/ions in this compound. Compounds with any structure can be represented by empirical formulae.

For example, consider ethane, with the molecule shown below:

圖片參考：http://www.school-for-champions.com/chemistry/images/hydrocarbon-ethane.gif

Each ethane molecule contains 2 carbon (C) atoms and 6 hydrogen (H) atoms. Therefore, the molecular formula of ethane is C2H6.
In ethane, the simplest ratio of (number of C atoms) : (number of H atoms) is 1 : 3. Therefore, the empirical formula of ethane is CH3.

empirical formula is find the amount of the element. molecular formula is the total number of relative atomic masses and it equal to molar masses.