Stoichiometry is the quantitative relationship among reactants and products in a chemical reaction.
Stoichiometry is derived from two Greek words: stoicheion (meaning "element") and metron (meaning "measure").
You often are required to calculate quantities of reactants or products.
Stoichiometry calculations are based on the fact that the number of atoms are conserved in a chemical reaction. The number and kinds of atoms before and after the reactions are always the same. Therefore we have to balance any chemical equation.
Stoichiometry is very simple when dealing with quantities in moles.
2 H2 + O2 -> 2 H2O
The coefficients in front of the chemical forumlas in the balanced chemical equation represents the number of moles.
The equation above should be read as " two moles of hydrogen gas reacts with two moles of oxygen gas to produce two moles of liquid water"
From the balanced chemical equation, a molar relationship can be found between each of the chemical species in the reaction. We call these relationships, molar ratios. These molar ratios can be used as conversion factors in stoichiometric calculations.