# 4.25: Stoichiometry: Introduction and Indicators

Learning Objectives
• Define stoichiometry.
• Identify the indicator information associated with stoichiometric molar relationships.

The first ten sections of this chapter presented and discussed three quantitative relationships involving the mole, which is a unit that relates a variety of chemically-significant numerical values and concepts to one another.  Avogadro's number, which has a value of 6.02 × 1023, corresponds to the number of individual atoms, ions, or molecules that are present within a substance.  "Component within" molar relationships indicate the relative ratios of the elements that are present within a compound or molecule.  Finally, atomic weight and molecular weight are molar quantities that relate to the mass of an element or a compound, respectively.

The final fundamental chemical quantity that will be discussed in this chapter is introduced below, and its associated equality pattern will be presented and applied in the following section.

## Stoichiometry

The previous four sections of this chapter described and applied the process of balancing chemical equations.  While the formulas of elements and compounds must change during a reaction, by definition, the relative quantity of each atom or ion involved in the reaction must be constant.  The subscripts that are present within a chemical formula are solely dependent on the elemental, ionic, or covalent nature of the corresponding substance and, therefore, cannot be altered to uphold the Law of Conservation of Matter, which mandates that no particles be created or destroyed in the course of a chemical reaction.  As a result, most reactions must be balanced, in order to account for any relative differences between the formulas of the reactants and products that are involved in the reaction.

Additionally, balancing coefficients indicate the ratio in which reactants must be present, in order to ensure a complete reaction, and dictate the number of product atoms or molecules that will be generated, relative to the amounts of reactants that are consumed.  Stoichiometry, which is the quantitative study of substances as they participate in chemical reactions, is a fundamental chemical concept.  Therefore, the balancing coefficients that are present in a reaction are chemically-significant quantities, and their corresponding chemical ratios are defined as molar standards.

## Stoichiometry Indicator Information

Stoichiometry studies the relative molar ratios of the atoms and molecules that participate in chemical reactions.  Unlike Avogadro's number, atomic weight, and molecular weight, which are indicated for use in a problem-solving context by the presence of specific key words, the indicator for stoichiometric molar relationships is highly generic:  In order to apply a stoichiometric equality, the given problem, which must be associated with a chemical reaction, must refer to two chemicals that participate in that reaction.

For example, consider the following chemical equation.

$$\ce{2 H_2O_2} \left( aq \right) \rightarrow \ce{2 H_2O} \left( l \right) + \ce{O_2} \left( g \right)$$

How many moles of O2 are produced from the decomposition of 7.53 moles of H2O2?

Because both of the chemicals that are referenced in the question, O2 and H2O2, are also present in the given reaction equation, a stoichiometric equality should be developed and applied when solving the problem.