The Mole

Mole is a term used in chemistry to represent the number 6.02 x 10^23.  Just as we use the word "dozen" to mean 12 objects, "mole" represents 6.02 x 10^23.

Amedeo Avogadro studied molecular theory in the early 19th century and built on the ideas of Dalton and Guy Lussac.  The number of particles in a mole was actually discovered later in the century and named in his honor.

Atoms and molecules are VERY small. Remember we measure their mass in atomic mass units, amu.  An amu is equal to 1/12th the mass of a carbon-12 atom or approximately the mass of a proton or neutron. The wonderful thing about the very odd number is it allows to work with measurable quantities.

One mole of atoms of any element is equal to its atomic mass (average mass number) in grams.  This is called molar mass.

The molar mass of a compound is simply the sum of the masses of each of its atoms.

water is H20
there are 2 Hydrogen and 1 Oxygen
therefore its molar mass is 2(1.0) + 1(16.0) or 18.0 grams/mole

This means that if you have 18.0 grams of water, you will also have 6.02 x 10^23 molecules of water.