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The Value of a Mole
The set value for a mole is equal to the number of atoms present in 12 pure grams of carbon-12, as long as the atoms are at rest and in an unbound state. Any substance can be measured using the mole system, but the actual amount of substances varies greatly according to other measurement systems. Because the molecular makeup is what is actually studied, moles are usually only used as an expression in the study of chemistry.
The Avogadro constant
The actual number of entities contained in a mole of substance is a number known as the Avogadro constant. This number is expressed as 6.02 x 10^23 --- an enormously large figure, and another reason it is mostly used only in chemistry with microscopic elements being quantified. In those terms, one mole is the amount of a substance containing this number of elementary entities.
Using the Mole
The advantage of using a mole is it allows you to quantify the amount of something when its individual constituents are too small to count. Because you know the chemical values of a substance, you can use mathematical equations to figure the amount it would take to equal one mole. For example, you can find the amount of the substance equalling one mole if you know the substance's mean molecular weight, expressed in atomic mass units. The mean molecular weight of many pure substances is known and easy to look up on a periodic table. For example, the mean molecular weight of pure water, H2O, is 18.015, meaning that 18.015 grams of water is equal to one mole.
Significance
The advantage and practicality of the mole is that it allows microscopic quantities to be expressed in the macroscopic physical world, as in the example with the water. Moles are used to measure the amount of substance and can represent the mass, volume or number of particles in an object.