Editing Stoichiometry (section)

== Definitions ==
{{Main|Amount of substance}}
A '''stoichiometric amount'''<ref>''What's in a Name? Amount of Substance, Chemical Amount, and Stoichiometric Amount'' Carmen J. Giunta Journal of Chemical Education 2016 93 (4), 583-586 {{doi|10.1021/acs.jchemed.5b00690}}</ref> or '''stoichiometric ratio''' of a [[reagent]] is the optimum amount or ratio where, assuming that the reaction proceeds to completion:
# All of the reagent is consumed
# There is no deficiency of the reagent
# There is no excess of the reagent.

Stoichiometry rests upon the very basic laws that help to understand it better, i.e., [[law of conservation of mass]], the [[law of definite proportions]] (i.e., the [[law of constant composition]]), the [[law of multiple proportions]] and the [[law of reciprocal proportions]]. In general, chemical reactions combine in definite ratios of chemicals. Since chemical reactions can neither create nor destroy matter, nor [[nuclear transmutation|transmute]] one element into another, the amount of each element must be the same throughout the overall reaction. For example, the number of atoms of a given element X on the reactant side must equal the number of atoms of that element on the product side, whether or not all of those atoms are actually involved in a reaction.<ref>{{Cite web |title=Stoichiometry of Chemical Reactions |url=https://web.ung.edu/media/chemistry/Chapter4/Chapter4-StoichiometryOfChemicalReactions.pdf}}</ref>

Chemical reactions, as macroscopic unit operations, consist of simply a very large number of [[elementary reaction]]s, where a single molecule reacts with another molecule. As the reacting molecules (or moieties) consist of a definite set of atoms in an integer ratio, the ratio between reactants in a complete reaction is also in integer ratio. A reaction may consume more than one molecule, and the '''stoichiometric number''' counts this number, defined as positive for products (added) and negative for reactants (removed).<ref name=GoldBookS06025>{{GoldBookRef |title=stoichiometric number, ''ν'' |file=S06025 }}</ref> The unsigned coefficients are generally referred to as the '''stoichiometric coefficients'''.<ref>{{cite web |last1=Nijmeh |first1=Joseph |last2=Tye |first2=Mark |title=Stoichiometry and Balancing Reactions |url=https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Modules_and_Websites_(Inorganic_Chemistry)/Chemical_Reactions/Stoichiometry_and_Balancing_Reactions#:~:text=The%20stoichiometric%20coefficient%20is%20the,product%20sides%20of%20the%20equation. |website=LibreTexts |date=2 October 2013 |access-date=5 May 2021}}</ref>

Each element has an [[atomic mass]], and considering molecules as collections of atoms, compounds have a definite [[molecular mass]], which when expressed in daltons is numerically equal to the [[molar mass]] in [[Gram|g]]/[[Mole (unit)|mol]]. By definition, the atomic mass of [[carbon-12]] is 12&nbsp;[[Dalton (unit)|Da]], giving a molar mass of 12&nbsp;g/mol. The number of molecules per mole in a substance is given by the [[Avogadro constant]], exactly {{physconst|NA|ref=no}} since the [[2019 revision of the SI]]. Thus, to calculate the stoichiometry by mass, the number of molecules required for each reactant is expressed in moles and multiplied by the molar mass of each to give the mass of each reactant per mole of reaction. The mass ratios can be calculated by dividing each by the total in the whole reaction.

Elements in their natural state are mixtures of [[isotope]]s of differing mass; thus, [[atomic mass]]es and thus molar masses are not exactly integers. For instance, instead of an exact 14:3 proportion, 17.04&nbsp;g of ammonia consists of 14.01&nbsp;g of nitrogen and 3&nbsp;×&nbsp;1.01&nbsp;g of hydrogen, because natural nitrogen includes a small amount of nitrogen-15, and natural hydrogen includes hydrogen-2 ([[deuterium]]).

A '''stoichiometric reactant''' is a reactant that is consumed in a reaction, as opposed to a [[catalysis|catalytic reactant]], which is not consumed in the overall reaction because it reacts in one step and is regenerated in another step.