The amount of substance is a fundamental unit in chemistry that quantifies the amount of matter present in a sample, regardless of its physical state. It is measured in moles ($\text{mol}$), and one mole represents a fixed, extremely large number of particles, known as Avogadro’s number. This concept allows for the calculation of molar mass, which provides the mass per mole of a substance. By determining the amount of substance, scientists can accurately relate bulk quantities of materials to the number of individual particles, facilitating stoichiometry calculations and enabling the prediction of chemical reactions and physical properties of matter.