The mole is the unit of measurement in the International System of Units (SI) for an amount of substance. The number of molecules per mole is known as Avogadro's constant and is defined such that the mass of one mole of a substance, expressed in grammes, is equal to the mean relative molecular mass of the substance.
Example: -1 mole of hydrogen molecules represents 6.022 × 1023 hydrogen molecules.
Balanced chemical reactions are balanced in terms of moles. A balanced chemical reaction gives equivalences in moles that allow stoichiometry calculations to be performed. Mole quantities of one substance can be related to mass quantities using a balanced chemical equation.
Stoichiometry is the branch of chemistry that deals with the relationship between the relative quantities of substances taking part in a chemical reaction. Let's write a general chemical reaction where there are two reactants, A and B that react together to form two products, C and D respectively.