Molecular and Empirical Formulas

• The molecular formula of a substance shows the actual number of atoms of each element present in a compound or molecule.
• The empirical formula of a substance is the simplest, whole number ratio of atoms of each element in a compound. This means that it doesn't tell you exactly how many atoms there are, just the ratio of the atoms of each element. 

  For example, the empirical formula for glucose is CH2O, which means that the ratio of carbon : hydrogen : oxygen is 1 : 2 : 1. Or in other words, for every 1 carbon atom, there are 2 hydrogen atoms and 1 oxygen atom. 
• Some examples of substances molecular formulas and empirical formulas 

  
• Looking at the molecular formula of ethane (C2H6), we can see that the ratio of C : H atoms is 2 : 6

  2. To get the empirical formula, we need to simplify this ratio as much as possible. You can simplify the ratio 2 : 6 by dividing both sides by 2 - which gives you 1 : 3

  3. This 1 : 3 ratio means the empirical formula for ethane is 1 carbon atom for every 3 hydrogen atoms.

  4. So the empirical formula for ethane is CH3

  How to find the molecular formula from the empirical formula and Mr
• To find the empirical formula from the molecular formula, all you do is find the simplest whole number ratio of the atoms in the compound.