Describing amounts of substances the mole

Chemists have adopted a special unit of measurement called the mole (abbreviation mol) to describe the amount of substance. A mole is defined as 6.022 136 7 X 1023 molecules or atoms. This is chosen to be equivalent to its molecular weight in grams. Thus 1 mol of sodium, which has an atomic weight of 23 (if one were to be very accurate, it would be 22.9898), weighs 23 g and contains 6.022 136 7 X 1023 molecules. This special number of particles is called the Avogadro number, in honour of the Italian physicist Amedeo Avogadro.

The mole is used because it always refers to the same amount of substance, in terms of the number of molecules, regardless of mass of the atoms involved. Thus a mole of a light element like sodium, and a mole of a heavy element like uranium (U), contain the same numbers of atoms.

Units of concentration are, for this reason, also expressed in terms of the mole. Thus a concentration is given as the amount of substance per unit volume as mol dm-3 (the unit of molarity) or per unit weight as mol kg-1 (molality). The latter is now used frequently in chemistry because it has a number of advantages (such as it does not depend on temperature). However, at 25°C, in a dilute solution, mol kg-1, mol dm-3 and mol l-1 are almost equivalent. Although mol l-1 is not an accepted SI unit it remains widely used in the environmental sciences, which is the reason we have decided to use it throughout this book.

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