# What does “stoichiometric amount” mean?

Can anyone inform me what the term stoichiometric amount means? I know what stoichiometry means:

The relationship between the relative quantities of substance taking part in a reaction or forming a compound.

But what is “stoichiometric amount” supposed to be?

• Stoichiometric just means element measure. With regard to a balanced chemical reaction equation, the reactants are in stoichiometric proportion if the reactants are in ‘just right’ proportion: no reactant is limiting or present in excess. In a sense, it is ‘least wasteful’. But, as one of the answers notes, real world situations may make it necessary to have non-stoichiometric conditions, e.g., synthesizing cisplatin from an expensive platinum reactant plus an inexpensive reactant. See wiki for cisplatin. Then the cheap reactant may be used in excess to avoid wasting the expensive reactant.
– Ed V
Feb 29, 2020 at 14:40

Sometimes, practical chemistry does not work like it's supposed to on paper. A perfectly balanced reaction equation may dictate that two reactants should be in a 1:1 stoichiometric ratio, but in practice, you get much better yields or reaction times if you use an excess of one.

When that happens, you are no longer using a stoichiometric amount, as specified by the balanced equation, but a super- or substoichiometric amount.

Welcome to chemistry in the real world.

IUPAC defines the term “stoichiometric amount” plainly yet rather vigorously on the basis of thermodynamic system [1] and employs it further in expression of chemical activity via chemical potential (emphasis mine):

2.1. The physico-chemical description of a component in a chemical system is based on two quantities:

(1) An extensive quantity: the amount of substance of the component added or removed in the process of formation of the system, positive when added, negative when removed. This quantity is sometimes called the stoichiometric amount of substance of the component in the system, symbol $$n.$$

(2) An intensive quantity: the chemical potential of the component in the system, symbol $$\mu.$$

[…]

The component added to the system may dissociate or react with other components to form a series of derived components and only a fraction of the original component may actually exist in a free form in the system. It is therefore essential to distinguish between the stoichiometric concentration and the substance concentration of the free form of the component in the system. Sometimes stoichiometric quantities are indicated by subscript $$(_\circ),$$ e.g. $$n_{\circ,\ce{B}},$$ $$c_{\circ,\ce{B}}.$$

The IUPAC Gold Book's definition of stoichiometry (even though there are several other definitions) also stresses out that stoichiometric relationship is merely a snapshot in time already for the simplest reaction such as $$\ce{A -> B -> C}$$. Тhe majority of chemical reactions don't have constant relationship between the amounts of substances, and the stoichiometric equation is just a crude approximation of the real-life scenario. Additionally, reactants not always combine in exact whole-number ratios. It is therefore imperative not to (pre)define stoichiometric amounts based solely on a single stoichiometric equation.

There are several consequential features [2]:

• It doesn't matter how such system reaches equilibrium or is “constructed” in general.
• Stoichiometric amount depends on the choice of elementary components, and, since stoichiometric coefficient of an elementary reaction can be negative, the stoichiometric amount can also change not only its value, but also its sign, too, depending on which other non-derived components are present.

The author Ole Siggaard-Andersen, M.D. being a clinical chemist added a simple remark on his website:

Specifying the stoichiometric amount of the components in a system is a prescription for the system…

Note the terms “stoichiometric amount” and “equivalent amount” are often liberally used interchangeably [3], and I suspect this is the meaning implied in your context (AoS is the amount of substance):

A stoichiometric amount of one reactant can mean the amount that will react completely with a given amount of another reactant, leaving no excess of either one. In this usage, the amount may be a mass, a volume, or an AoS, and the adjective stoichiometric refers not to stoichiometry in general but to stoichiometric equivalence in particular. Thus, this sense is synonymous with equivalent amount.

I would recommend to read an entire Giunta's paper as during its proposition of a different meaning to the term suggesting it as a replacement for AoS, it also outlines pros and cons of several related terms, elaborates why “number of moles” is illiterate and does many other things right.

### References

1. Siggaard-Andersen, O.; Durst, R. A.; Maas, A. H. J. Physicochemical Quantities and Units in Clinical Chemistry with Special Emphasis on Activities and Activity Coefficients (Recommendations 1983). Pure Appl. Chem. 1984, 56 (5), 567–594. DOI: 10.1351/pac198456050567. (Free Access)
2. Siggaard-Andersen, O. Stoichiometric Concentration and Chemical Potential. Scand. J. Clin. Lab 1977, 146, 7–13. DOI: 10.3109/00365517709098926.
3. Giunta, C. J. What’s in a Name? Amount of Substance, Chemical Amount, and Stoichiometric Amount. J. Chem. Educ. 2016, 93 (4), 583–586. DOI: 10.1021/acs.jchemed.5b00690.

The amount of a substance produced or needed in the reaction of a specific amount of another substance with respect to a particular formula. It’s the amount your stoichiometric ratios tell you to use/expect.

Let’s take the simple example of the combustion of hydrogen $$\ce{H2}$$ in oxygen $$\ce{O2}$$. The equation of the reaction is: $$\ce{2 H2 + O2 -> 2 H2O}$$ It means that $$2$$ moles of $$\ce{H2}$$ $$(2\cdot2\ \mathrm g$$) react with $$1$$ mole ($$32\ \mathrm g$$) of $$\ce{O2}$$. These masses are in a ratio $$4:32=1:8$$. All mixtures containing $$\ce{H2}$$ and $$\ce{O2}$$ in a proportion $$1:8$$ are said to be “in stoichiometric amounts”.

If you use $$0.7\ \mathrm g$$ $$\ce{H2}$$ and $$5.6\ \mathrm g$$ $$\ce{O2}$$ to produce water, or if you have reacted $$1.10\ \mathrm g$$ $$\ce{H2}$$ and $$8.80\ \mathrm g$$ $$\ce{O2}$$, the components of the gas mixture are said to be “in stoichiometric amounts”.