What is defined as one mole of a substance?

One mole of a substance is defined as an amount containing approximately 6.02 x 10²³ particles, which is known as Avogadro's number. This fundamental concept in chemistry establishes that regardless of the type of substance—whether it’s atoms, molecules, ions, or other particles—one mole corresponds to this specific quantity. This definition allows chemists to count and quantify amounts of substances in a consistent manner, facilitating calculations that involve chemical reactions and stoichiometry.

The other options do not accurately define what a mole is. For instance, a substance's mass at room temperature can vary based on various factors and does not provide a standard measure related to the number of particles. Similarly, the volume of a substance in liters is not a fixed measure for a mole, since the volume can change depending on conditions like temperature and pressure. The total mass of all reactants relates to the law of conservation of mass in a chemical reaction, but it does not directly define a mole either. Hence, the precise definition pertaining to the number of particles is what makes the first option correct.