mole
/ mohl /
A grocer counts eggs by the dozen because twelve is a handy bundle. A chemist counts atoms and molecules by the mole because atoms are so absurdly small that you need an enormous bundle to make a weighable pile. One mole is just a specific, very large count of particles — the chemist's version of the dozen.
Precisely, a mole is the amount of a substance that contains exactly 6.02214076 × 10²³ elementary entities — atoms, molecules, ions, or whatever you specify. That number is the Avogadro constant. The mole is the official SI unit for amount of substance, and its magic is that one mole of a substance has a mass in grams equal to its formula's mass in atomic mass units, so weighing becomes counting.
The mole matters because reactions happen particle by particle, in whole-number ratios, but we work in the laboratory by weight. The mole is the bridge: it lets you translate a balanced equation, written in particles, into grams you can actually measure on a balance. Always say what you have a mole of — a mole of oxygen atoms is half as heavy as a mole of oxygen molecules.
Eighteen grams of water is one mole — about a tablespoon — yet it holds roughly six hundred thousand billion billion molecules, far more than the number of stars in the observable universe.
A spoonful of water, an astronomical count of molecules — that bundle is one mole.
A mole is a count, like a dozen, not a mass. The gram-mass of one mole differs from substance to substance because their particles weigh different amounts; only the number of particles is always the same.