THE MOLE
Since atoms are so small, any sensible laboratory quantity of substance must contain a huge number of atoms:
1 litre of water contains 3.3 x 10^{25} molecules.
1 gram of magnesium contains 2.5 x 10^{22} atoms.
100 cm^{3} of oxygen contains 2.5 x 10^{21}molecules.
Such numbers are not convenient to work with, so it is necessary to find a unit of “amount” which corresponds better to the sort of quantities of substance normally being measured. The unit chosen for this purpose is the mole. The number is chosen so that 1 mole of a substance corresponds to its relative atomic/molecular/formula mass measured in grams. A mole is thus defined as follows:
A mole of a substance is the amount of that substance that contains the same number of elementary particles as there are carbon atoms in 12.00000 grams of carbon12. 
One mole of carbon12 has a mass of 12.0g.
One mole of hydrogen atoms has a mass of 1.0g.
One mole of hydrogen molecules has a mass of 2.0g.
One mole of sodium chloride has a mass of 58.5g.
The number of particles in one mole of a substance is 6.02 x 10^{23}. This is known as Avogadro’s number, L or N_{A}
Thus when we need to know the number of particles of a substance, we usually count the number of moles. It is much easier than counting the number of particles.
The number of particles can be calculated by multiplying the number of moles by Avogadro’s number. The number of moles can be calculated by dividing the number of particles by Avogadro’s number.
(Number of particles) = (number of moles) x L

The mass of one mole of a substance is known as its molar mass, and has units of gmol^{1}. It must be distinguished from relative atomic/molecular/formula mass, which is a ratio and hence has no units, although both have the same numerical value.
The symbol for molar mass of compounds or molecular elements is m_{r}. The symbol for molar mass of atoms is a_{r}.
Mass (m), molar mass (m_{r} or a_{r}) and number of moles (n) are thus related by the following equation:
MASS = MOLAR MASS X NUMBER OF MOLES or m = m_{r} x n

Mass must be measured in grams and molar mass in gmol^{1}.