Which unit in chemistry is often used to express the quantity of a substance?

The mole is the fundamental unit in chemistry used to express the quantity of a substance. It is defined as the amount of substance that contains as many entities (such as atoms, molecules, or ions) as there are in 12 grams of carbon-12. This definition links the microscopic world of atoms and molecules with the macroscopic measurements of mass, allowing chemists to easily quantify and compare amounts of different substances.

Using moles provides a way to bridge the gap between the atomic scale and the laboratory scale, enabling calculations for chemical reactions, concentrations, and stoichiometry. Because reactions occur between defined numbers of entities, understanding and applying the concept of moles is essential for precise measurement and interaction of substances in chemical processes.

While grams are a unit of mass and liters measure volume, they do not directly reflect the quantity of substances in chemical terms as the mole does. An atom is a particle and not a unit for measuring quantities on a scale used in chemistry, making it less useful for expressing amounts of substances in laboratory contexts.