Avogadro's number:

Let assume one atom of oxygen and one atom of sulfur, and contrast their atomic weights.

Oxygen's atomic weight = 15.999 amu

Sulfur's atomic weight   = 32.06 amu

The sulfur atom weighs approximately twice as much as the oxygen atom. (32.06 ÷ 15.99 ≡ 2)

Since the sulfur atom weighs twice as much as an oxygen atom, single gram sample of oxygen contains twice as several atoms as a one gram sample of sulfur. Therefore, a two gram sample of sulfur contains the similar number of atoms as a one gram sample of oxygen.

From this earlier instance, one may suggest in which a relationship exists among the weight of a sample and the number of atoms within the sample. Actually, scientists have determined in which there is a definite relationship among the number of atoms within a sample and the sample's weight. Experimentation has display that, for any element a sample containing the atomic weight in grams contains 6.022 x 1023 atoms. Therefore 15.999 grams of oxygen carry 6.022 x 1023 atoms and 32.06 grams of sulfur carry 6.022 x 1023 atoms.  This number (6.022 x 1023) is called as Avogadro's number.  An importance of Avogadro's number to chemistry should be clear. That represents the number of atoms in X grams of any element, whereas X is the atomic weight of the element. It allows chemists to predict and use exact amounts of elements required to cause desired chemical reactions to occur.