A mole is a unit of measurement used in chemistry to express amounts of a substance. One mole contains approximately 6.022 x 10^23 particles, which can be atoms, molecules, or ions. This number is known as Avogadro's number and helps scientists quantify and compare different substances in chemical reactions. Moles are essential for understanding stoichiometry, which is the calculation of reactants and products in chemical reactions. By using moles, chemists can determine how much of each substance is needed or produced, making it easier to conduct experiments and create new compounds.