Mole Concept

Chemists constantly need to talk about huge numbers of atoms or molecules -- literally hundreds of billions of trillions of them in even a small sample -- and counting them one by one is obviously impossible. The solution is the same trick we use in everyday life: give a large, fixed number of items its own convenient name. Just as a "dozen" always means 12 items (12 roses, 12 balls, 12 apples) regardless of what is being counted, chemistry defines the mole as its own counting unit.

Definition. One mole is the amount of substance of a system that contains as many elementary particles (atoms, molecules, ions, electrons, or any other specified particle) as there are atoms in exactly 12 g of the carbon-12 isotope.

This definition is confirmed experimentally by Table 1.2: dividing the mass of a sample by the mass of a single atom/molecule of that substance always gives the same number, 6.022 × 10²³, whether the sample is 12 g of carbon-12, 180 g of glucose, 294.18 g of potassium dichromate, or 158.03 g of potassium permanganate. Because this count keeps recurring, it is given its own name -- exactly as "dozen" always means 12, "mole" always means 6.022 × 10²³ entities (atoms, molecules or ions). …