A mole is just a chemist's dozen, only 602 sextillion times bigger
A mole works exactly like a dozen or a pair: a name for a set number of things. The catch is the number. One mole holds precisely 602,214,076,000,000,000,000,000 particles, because atoms are so tiny that trillions of trillions are needed to make a lump you can weigh on a lab bench.
The mole, symbol mol, is the SI base unit for amount of substance. Since the 2019 revision of the SI it is defined as exactly 6.02214076 times ten to the 23rd elementary entities, whatever they happen to be: atoms, molecules, ions, ion pairs or even protons. That figure is the Avogadro number. Attach the unit per mole to it and you get the Avogadro constant, which was pinned down by counting the atoms of silicon-28 in a single crystal.
The old definition tied the mole to the number of atoms in 12 grams of carbon-12. That choice created a convenient coincidence: a compound's molar mass in grams per mole matched its molecular mass in daltons. Under the new definition the match is no longer exact, but it holds so closely that chemists can still rely on it.
Counting by moles makes chemical equations readable. Two hydrogen molecules combining with one oxygen molecule to give two water molecules also means 2 moles of hydrogen plus 1 of oxygen yield 2 moles of water. Ten moles of water and ten moles of mercury hold the same number of particles, one mercury atom for each water molecule, even though their masses and volumes differ completely. Concentrations are usually given in moles per litre.
What counts as an entity depends on convention. Dissolved molecules drift independently and are easy to tally. In a diamond, the whole crystal is effectively one giant molecule, so chemists count atoms instead. Industry often scales up to kilomoles, and because a cubic metre is a thousand litres, a concentration expressed in kilomoles per cubic metre has the same numerical value as moles per litre.
Source: Mole (unit)
