A chemical formula shows shows the number of different atoms in one molecule of a chemical compound. An example of a molecular formula is H2O2 or hydrogen peroxide. What makes this a molecular formula is that it is composed of two non-metals: hydrogen and oxygen. A mixture of individual hydrogen and oxygen molecules is highly combustable in fire, but together they form water which puts out fire. and then gets hot.
http://simple.wikipedia.org/wiki/Molecular_formula