Chemical Quantities: Chemical Equations

A chemical equation represents a chemical reaction. It shows the reactants (the substances that react) on the left-hand side and the products (the substances that are produced) on the right-hand side.
Each substance in a chemical equation is represented by its chemical symbol or formula, which shows how many atoms of each element are present.
In a balanced chemical equation, the number of atoms of each element in the reactants is equal to the number of atoms of that element in the products. This is due to the law of conservation of mass, which states that matter (in this case, atoms) cannot be created or destroyed in a chemical reaction.
For example, the equation for the reaction of hydrogen with oxygen to form water is 2H2 (g) + O2 (g) → 2H2O (l). This means that two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of water.
Coefficients (the numbers in front of the chemical symbols or formulas) are used to balance chemical equations. They indicate the number of moles of a substance.
The state of the substances - solid (s), liquid (l), gas (g) or aqueous dissolved in water (aq) - is often shown in brackets after the chemical formula.
The mole is the unit of amount in chemistry. One mole of a substance contains the same number of entities (atoms, molecules or ions) as there are atoms in 12 grammes of carbon-12.
Avogadro’s number, which is approximately 6.02 x 10^23, is the number of entities in a mole. It is usually denoted by the symbol N(A) or simply N.
Molar mass, measured in grammes per mole (g/mol), is the mass of one mole of a substance. It is numerically equal to the relative formula mass (or relative molecular mass) of the substance, which is the sum of the relative atomic masses of the atoms in the formula.
Stoichiometry is the study of the quantitative relationships between the reactants and products in a chemical equation, such as the ratios of moles of substances. For example, in the equation 2H2 + O2 → 2H2O, the stoichiometric ratio of hydrogen to oxygen to water is 2:1:2.
Calculations involving chemical quantities - moles, masses, volumes, concentrations and energies - are an important part of understanding chemical reactions and are often needed in experimental work.