Stoichiometry refers to the relationship between the amounts of substances of the reactants and the amount of substances of the formed products. This tag should be used for specific questions about chemical reaction equations.

Stoichiometry refers to the relationship between the amounts of substances of the reactants and the amount of substances of the formed products. It is closely related to chemical reaction equations. As the total mass of the reactants has to equal the total mass of the products, it also reflects the law of conservation of mass.

Deriving from this it is possible to calculate the amount of product that will be formed, if the amount of reactants is known, and vice versa. A reaction can be classified as stoichiometric if it follows simple integer ratios, i.e. no excess of reactants or products. $$\ce{NaCl_{(aq)} + AgNO3_{(aq)} -> Na_{(aq)}^{+} + NO3_{(aq)}^{-} + AgCl_{(s)} v}$$

Some reactions are more complicated and involve several reaction paths or mechanisms, but the overall stoichiometry has to be preserved.

The precise IUPAC Definition of Stoichiometry:

The term refers to the relationship between the amounts of substances that react together in a particular chemical reaction, and the amounts of products that are formed. The general stoichiometric equation: $$\ce{a A + b B + \dots -> \dots + y Y + z Z}$$ provides the information that $\ce{a}$ moles of $\ce{A}$ reacts with $\ce{b}$ moles of $\ce{B}$ to produce $\ce{y}$ moles of $\ce{Y}$ and $\ce{z}$ moles of $\ce{Z}$. The stoichiometry of a reaction may be unknown or may be very complex. For example, the thermal decomposition of acetaldehyde yields mainly methane and carbon monoxide, but also a variety of minor products such as ethane, acetone, and diacetyl. The stoichiometric equation $$\ce{CH3CHO -> CH4 + CO}$$ is therefore only an approximate one. Even when the overall stoichiometry of a reaction is well defined, it may be time-dependent in that it varies during the course of a reaction. Thus if a reaction occurs by the mechanism $\ce{A -> X > -> Y}$, and $\ce{X}$ is formed in substantial amounts during the course of the process, the relationship between the amounts of $\ce{A}$, $\ce{X}$ and $\ce{Y}$ will vary with time, and no one stoichiometric equation can represent the reaction at all times.

Further reading:

history | excerpt history