A chemical equation is a visual representation of a chemical reaction. A typical chemical equation follows the form
We can see the relationship between O2 and H2O: for every one mole of O2, two moles of H2O are produced. Therefore our ratio is one mole of O2 to two moles of H2O. Now, if we assume abundant hydrogen, and we have two moles of O2, then we can calculate:
The stoichiometric conversion factors are reaction-specific and require that the reaction be balanced. Therefore, the above steps must be followed for every single reaction you encounter.