Moles. A collection of beakers, each containing one mole of a substance. On the balance is a flask containing one mole of copper (II) sulphate pentahydrate. One mole of any sample contains the same number of molecules or atoms, but the weight and volume are different. The mole, one of the base international (SI) units, was defined in 1971 in relation to the number of atoms in 12 grams of the most abundant isotope of carbon. This is known as Avogadro's number, 6.02 x 10^23.