Stoichiometry is the quantitative study of the relative amounts of reactants and products in chemical reactions; gas stoichiometry involves chemical reactions that produce gases. Stoichiometry is based on the law of conservation of mass, meaning that the mass of the reactants must be equal to the mass of the products. This assumption can be used to solve for unknown quantities of reactants or products.
Stoichiometric Calculations Involving Ideal Gases at STP
Stoichiometric calculations involving gases allow us to convert between mass, number of moles, and most importantly, volume of gases. The following relationship makes this possible:
 1 mole of any gas at standard temperature and pressure (273 K and 1 atm) occupies a volume of 22.4 L.
While the above relationship is an estimation, it is a relatively good approximation at STP, and can be used reliably in calculations.
Example
 According to the above reaction, what volume of NO_{2}(g) is produced from the combustion of 100 g of NH_{3}(g), assuming the reaction takes place at standard temperature and pressure?
 From the periodic table, we can determine that the molar mass of ammonia, NH_{3}(g), is 17 g/mol, and perform the following stoichiometric calculation:

$\left(\frac{\text{100 g }NH_3}{ }\right)\times\left(\frac{\text{1 mol }NH_3}{\text{17 }g}\right)\times \left(\frac{\text{4 mol }NO_2}{\text{4 mol }NH_3}\right)\times \left(\frac{\text{22.4 }L}{\text{1 mol }NO_2}\right)=\text{132 L }NO_2(g)$
Note the final conversion factor. Because we are told that the reaction takes place at STP, we can relate volume, 22.4 L, to 1 mol NO_{2}.