What defines a mole of a substance?

A mole of a substance is defined as the amount that contains a specific number of particles, which is Avogadro's number, approximately ( 6.022 \times 10^{23} ) particles. This number can represent atoms, molecules, ions, or other entities, depending on the substance in question. The concept of a mole is fundamental in chemistry because it allows chemists to count the number of particles in a sample by weighing it. It serves as a bridge between the microscopic world of atoms and molecules and the macroscopic world we can measure and manipulate in the laboratory.

Understanding the mole is crucial for stoichiometry, where it facilitates calculations involving chemical reactions, such as determining how much reactant is needed or how much product can be formed from given amounts. Thus, the correct answer emphasizes the mole's role as a unit of amount in the context of particle quantity, making it a fundamental concept in chemistry.