# What does “stoichiometric amounts” mean?

Can anyone inform me what the term stoichiometric amounts means? I know what stoichiometry means. It is the:

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 '20 at 14:40

## 3 Answers

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.

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”.

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.