Mole (unit)

In chemistry and physics, the mole is an SI base unit of amount of substance. The unit is abbreviated mol. The word "mole" is derived from "gram molecular weight", the original term. Industrial chemists also used a "kilogram molecular weight", equal to 1 kmol.

Loosely speaking, the mole may be defined for a pure substance, consisting of one kind of molecules, as the amount of substance that has a mass in grams equal to the molecular weight of the molecule. For instance, the molecular weight of the water molecule (H₂O) is 18.02, and therefore one mole of water is 18.02 grams of water.

The definition of a mole extends to different entities: molecules, ions, atoms, electrons - any elementary substance in chemistry. The SI definition reads: the mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of ¹²C (carbon-12).

The definition of the mole leads to the definition of Avogadro's constant: N_A is the numner of entities in one mole. N_A ≈ 6×10²³/mol

One mole of an ideal gas occupies 22.414 litres at "standard temperature and pressure" (273.15K = 0 ⁰C and 101.325 kPa = 1 atm). Real gases will deviate slightly from this amount.

The total mass of an amount of substance is the sum of the masses of its entities. For example, consider a pure substance B of entities with molecular mass M(B) u ( u is unified atomic mass unit). Recalling that

1 u = 1/N_A gram,

we find that one mole of B weighs N_A × M(B) u = M(B) gram. For example, N_A molecules of water (B = H₂O) have mass 18.02 gram.

Let us give another example. The standard atomic weight of magnesium M(Mg) = 24.3050 u, which is the mass of a magnesium atom averaged over its isotopes weighted by natural abundance. One mole of magnesium is N_A × M(Mg) u = 24.3050 10⁻³ kg. This example shows that the atomic mass of any element can be interpreted in two ways: (1) the average mass of a single atom in unified atomic mass units (u) or (2) the average mass of a mole of atoms in grams. For magnesium, (1) the average mass of a single magnesium atom is 24.3050 u or (2) the average mass of a mole of magnesium atoms is 24.3050 gram.

Futher examples: a mole of hydrogen molecule, standard atomic weight of H is 1.00794, has the mass 2×1.00794 = 2.01588 ⁻³ kg. A mole of oxygen, standard atomic weight 15.9994, has the mass 31.9988 10⁻³ kg.

To explain the usefulness of the mole concept we consider the following example of a chemical reaction:

2 A + 6 B → 2 AB₃

This equation is expressed in numbers of atoms and molecules, which are impractical to measure or count directly. However, if we multiply the equation by Avogodro's number, it is expressed in macroscopic quantities. The equation tells us then that 2 moles of A react with 6 moles of B to give 2 moles of AB₃.

In general, the molecular masses of the compounds A [ = M(A)] and B [ = M(B)] are known and hence also is the molecular mass of AB₃ [ = M(AB₃)]. The reaction equation can be translated thus: 2M(A) gram of A reacts with 6M(B) gram of B to give 2M(AB₃) gram of AB₃.

A real life example:

2H₂ + O₂ → 2H₂O

Using rounded numbers: 4 gram H₂ reacts with 32 gram O₂ giving 36 gram H₂O.

Notes

Sources

• mole. Sizes.com (2006-11-07). Retrieved on 2007-05-11.