Stoichiometry rests upon the law of definite proportions (i.e., the law of constant composition) and the law of multiple proportions. In general chemical reactions will combine definite ratios of chemicals.
Stoichiometry is often used to balance chemical equations. For example, the two diatomic gases hydrogen and oxygen can combine to form a liquid, water, in an exothermic reaction, as described by Equation 1.
The term stoichiometry is also often used for the molar proportions of elements in stoichiometric compounds. For example, the stoichiometry of hydrogen and oxygen in H2O is 2:1. In stoichiometric compounds, the molar proportions are whole numbers (that is what the law of multiple proportions is about).
Compounds for which the molar proportions are not whole numbers are called nonstoichiometric compounds. Such compounds can be produced by sputtering in a plasma. They are not in chemical equilibrium.
Solids that actually are a mixture of very small crystallites of compounds of different stoichiometry also have been loosely called nonstoichiometric compounds. This is incorrect and probably due to the difficulty in observing the very small crystallites. If a solid was misinterpreted as homogeneous, it was consequently misinterpreted as nonstoichiometric.
Petrol's stoichiometric mixture is 14.6:1, although engines are often run slightly on one side or the other of this perfect mix for a variety of reasons. Other common fuels include: