"One mole contains exactly elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, and is known as the Avogadro number."
It is expressed in the unit .
One mole of Water :
Water is made up of two hydrogen atoms and one oxygen atom.
One mole of the water molecule has this can be calculated by adding the masses of a number of atoms involved in the formation of the water molecule i.e. 2 hydrogen atoms and one oxygen atom.