Stoichiometry

Stoichiometry is the part of chemistry that deals with the quantitative relationships between the substances taking part in a chemical reaction. It rests on the law of conservation of mass, which states that atoms are neither created nor destroyed in a reaction, only rearranged. A balanced chemical equation expresses this by showing the same number of atoms of each element on both sides; the coefficients placed in front of the formulas give the fixed ratios in which substances combine and form.

Central to these calculations is the mole, the chemist's unit for counting particles. Because individual atoms and molecules are far too small and numerous to count directly, the mole represents a fixed, very large number of particles, allowing measurable masses to be linked to definite numbers of atoms or molecules. Using the molar mass of a substance, a mass in grams can be converted to moles, and the coefficients of a balanced equation then relate the moles of one substance to the moles of another, so the amount of product from given reactants can be predicted.

Many chemical equations involve oxidation and reduction, in which electrons are transferred between substances. Balancing such redox equations requires tracking oxidation states, the assigned charges that show how electrons are distributed, and writing separate half-reactions for the loss and gain of electrons before combining them. Stoichiometry underpins much of practical chemistry, from determining how much of a reactant is needed, to calculating yields, to analysing the composition of compounds.

Frequently asked questions

What is the mole in chemistry?

A unit representing a fixed, very large number of particles. It lets chemists relate the measurable mass of a substance to a definite number of atoms or molecules.

Why must a chemical equation be balanced?

Because atoms are conserved in a reaction. Balancing ensures the same number of each kind of atom appears on both sides and gives the correct ratios for calculations.

What is a redox reaction?

One in which electrons are transferred between substances, so some are oxidised (lose electrons) and others reduced (gain electrons), tracked using oxidation states.