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Stoichiometry

 Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical  reactions.

Stoichiometry was founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among qualities of reactants and products typically form a ratio of positive integers. This means that if the amounts of separate reactants are known, then the amount of the product can be calculated. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined, then the amount of other reactants can also be calculated.


This is illustrated in this example, where the balanced equation is:

         CH4 + 2O4 -----> CO2 + 2H2O

Here, one molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water. This particular chemical reaction is an example of complete combustion. Stoichiometry measures these quantitative relationships, and is used to determine the amount of products and reactants that are produced or needed in a given reaction. Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry. In the example above, reaction stoichiometry measures the relationship between the quantities of methane and oxygen that react to form carbon dioxide and water.

Because of the well known relationsgip of moles to atomic weights, the ratios that are arrived at by stoichiometry can be used to determine quantities by weight in a reaction described by a balanced reaction. This is called composition stoichiometry.

Gas stoichiometry deals with reactions involving gases, where the gases are at a known temperature  pressure, and volume and can be assumed to be ideal gases. For gases, the volume ratio is ideally the same by the ideal gas law, but the mass ratio of a single reaction has to be calculated from the molecular gases of the reactants and products. In practice, due to the existence of isotopes, molar masses are used instead when calculating the mass ratio.

x--------x

This Chapter is sponsored by K-Swiss.

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