1. Introduction and basic guide

What is stoichiometry?

Stoichiometry is a very basic chemistry concept. It is just a way of measuring or determining the amount of each substance that is involved in a reaction (reactants), and the amount of products that are generated.

Before actually running a reaction in the lab, a chemist needs to figure out what is the number of molecules of each reagent is required for the reaction to proceed. For this purpose, we use a unit called “mole”. The mole is the base unit of “amount of substance”. One mole accounts for 6.022·1023 molecules. We need it to be a huge number, since there is a huge number of molecules in each gram of any reagent of a reaction.

The stoichiometry of a reaction is the measurement of the relative quantities (or equivalents), measured in moles, of the reactants that are involved in the reaction.

For instance, each 2 molecules (or 2 moles) of hydrogen gas (H2) react with 1 molecule (or 1 mole) of oxygen gas (O2), to generate 2 molecules (or 2 moles) of water (H2O):

2 H2 (gas) + 1 O2 (gas) → 2 H2O (liquid)

So if we were to perform this (rather inpractical, using two expensive, difficult to handle gases, to get a cheap easy to find product such as water) reaction in a lab, we would have to mix together 2 moles of hydrogen per mole of oxygen. This means using 2 equivalents (equiv) of hydrogen respect to the amount of oxygen. In this case, while handling gases, the number of moles of each can be controlled by establishing a partial pressure for each of them.

In case of more common solid reagents, we can consider a hypotetical synthesis of sodium carbionate from carbonic acid. We need 2 equivalents of sodium hydride (NaH) per mole of carbonic acid employed (twice as many molecules of NaH than molecules of carbonic acid).

stoichiometry of sodium carbonate synthesis
Synthesis of sodium carbonate from carbonic acid

This would yield 1 equivalent, or 1 mole of sodium carbonate, and also release 2 equivalents (or 2 moles) of hydrogen gas as a byproduct.

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