Mole (unit)

The mole is a unit of measurement in chemistry, which relates the number of entities (atoms, molecules, or ions) to the mass of the material. The word "mole" is shortened from "gram molecular weight", the original term. Industrial chemists also used a "kilogram molecular weight", equal to 1000 mole.

Chemical reaction formulae are expressed in molecules and atoms, which are impractical to measure or count directly. However, as the atomic weight of any given atom is constant, and generally known, it is possible to quantify the amount of substance by measuring the weight in grams, and dividing by the molecular weight of the molecule (the sum of the atomic weights of the individual atoms in the molecule), which yields the number of moles in the sample weighed.

Technically, a mole is defined in the SI as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. A mole of a substance has a mass in grams which is equal to the mass of a single atom or molecule of the substance in atomic mass units. The number of entities (atoms or molecules) of a substance in one mole is known as Avogadro's constant, which is approximately 6.022 141 5 × 10²³.

One mole of an ideal gas occupies 22.414 litres at "standard temperature and pressure" (273.15K and 101.325 kPa).

Another way to phrase this explanation is that a mole is the molecular mass in grams. Using the carbon-12 isotope, a mole of carbon-12 is 12 grams. For purposes of illustration, in answer to the question How many carbon-12 atoms are needed to have a mass of exactly 12 grams? that number, Avogadro's number, is the number of carbon-12 atoms in 12 grams of carbon-12. The abbreviation for Avogadro’s number is NA. NA is defined by:

NA x (mass of carbon-12 atom) = 12 g

In other words, the number of entities (atoms or molecules) of a substance in one mole is Avogadro's constant. This also applies to all other such entities. The atomic mass of magnesium is 24.305 amu, the average isotopic mass of magnesium as it naturally occurs. The molar mass of magnesium in grams can be derived in the same way. From the equation NA x (mass of atom) = X grams” we get 1 amu = 1g/NA or 1 amu = 1.66054x10^-24g. Using this we calculate for magnesium: NA x 24.305 amu x (1.66054x10^-24 g/amu) = 24.305 g

This means that a mole of magnesium atoms has a mass of 24.305 grams. 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 atomic mass units (amu) or (2) the average mass of a mole of atoms in grams. For magnesium, (1) the average mass of a single magnesium atom is 24.305 amu or (2) the average mass of a mole of magnesium atoms is 24.305 g;

Correspondingly: a mole of hydrogen, molecular mass 1.0079 is 1.0079 grams, a mole of lithium, molecular mass 6.94, is 6.94 grams. Molecules also have the same measure. A molecule of water, H₂O is two hydrogen (at 2 times 1.0079) and one oxygen (15.9994) for a combined molecular mass of 18.0152. So a mole of water would contain 18.0152 grams.

Notes

Sources

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