Stoichiometric calculations are based on the following two major principles.

• The composition of any substance in the chemical equation should be expressed by a definite formula.
• The law of conservation of mass must be obeyed (the mass of reactants equals the mass of products).

The Mole Concept

We may group different items in dozens or grosses. Analogous to the grouping of items, particles like atoms and molecules are grouped in moles in chemistry.

The mole provides the basis for relating masses in grams to number of atoms, molecules or formula units.

The mole is the amount of a substance that contains the same number of particles as the number of atoms in exactly 12 g of carbon-12. 12 g of carbon-12 contains an Avogadro’s number of carbon-12 atoms.

Avogadro’s number = 6.022 × 10²³

A mole of any substance is a group containing 6.022 × 10²³ particles of that substance whether the particles are atoms, molecules or ions.

The mass of one mole of atoms, one mole of molecules and one mole of an ionic compound is equal to the atomic mass, molecular mass and formula mass expressed in grams, respectively.

For example:

• One mole of carbon atoms has a mass of 12 g.
• One mole of water molecules has a mass of 18 g.
• One mole of sodium chloride has a mass of 58.5 g

The mass of one mole of atoms, molecules or a compound is called molar mass.

Calculations based on formulas

From a balanced chemical equation, it is possible to determine the:

• Number of moles of each reactant and product; and
• Relative mass of each of the reactants and products

For example, in the reaction of hydrogen with oxygen to produce water, 2 moles of H₂ combines with 1 mole of O₂ to yield 2 moles of H₂O.

The equation also tells us 4 g of hydrogen reacts with 32 g of oxygen to produce 36 g of water. This can be further interpreted as follows:

Calculations based on chemical equations (stoichiometric problems) are classified into mole–mole problem, mass–mass problems and mass–mole, mole atom, particle, molecule problems.