Mole (unit)

The mole is a fundamentally important unit of measurement in chemistry. If we have a sample of a certain element, then we might ask how much of the element is present in the sample? When the answer is given in moles, we can calculate exactly how many molecules of the element are present. If we then use that sample in a chemical reaction in combination with known amounts of other elements, we can calculate (using chemical formulae), exactly how much of a given compound will result from the reaction.

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.

Another way to phrase this is that a mole is the molecular mass in grams. Using the carbon-12 isotope, a mole of carbon-12 is 12 grams.

We pose 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 of carbon-12. The abbreviation for Avogadro’s number is NA. NA is defined by:

NA x (mass of carbon-12 atom) = 12 g”

So, 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.

This also applies to other entities, magnesium for example. The atomic mass of magnesium is 24.305 amu, the average isotopic mass of magnesium as it naturally occurs. What is the molar mass of magnesium in grams? From the equation NA x (mass of atom) = X grams” we get 1 amu = 1g/NA or 1 amu = 1.66054x10^-24 g. 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.

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

The word "mole" is shortened from "gram molecular weight", the original term. Industrial chemists also used a "kilogram molecular weight", equal to 1000 mole.

Notes

Sources

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