Significance of Avogadro's Number

Avogadro's number is the number of atoms,molecules or ions in one mole of a substance. It is the number of atoms in exactly 12 grams of carbon-12 or 16 grams of Oxygen-16. This experimentally determined value is approximately 6.02214179×1023 particles per mole.

We need to look at another parameter called AMU (atomic mass unit).One AMU is the average of the proton rest mass and the neutron rest mass. This is approximately  1.67377 x  10^-24 grams . 

The mass of a mole of substance is called the molar mass of that substance. The molar mass is used to convert grams of a substance to moles and is used often in chemistry.

There are 6 protons and six neutrons in carbon-12.

          so the molar mass of carbon-12 atom = 
           12 x 1.67377 x 10^-24 x 6.02214179×1023

2323

                        = 12.09 grams 

That is 1 mole of carbon is equivalent 12 grams of carbon-12 and contains Avogadro's number of atoms of Carbon-12. An element's atomic mass is listed as the average of all its isotopes on earth and can found in the periodic table. 

Let us calculate the molar mass of water   H2O

Symbol Element  Atomic weight   Atoms  

H Hydrogen 1.00794             2  

O Oxygen        15.9994             1  

Molar mass of H2O = 2 x 1.00794 + 1 X 15.9994 =18.01528 grams/mole

So one mole of any substance contains Avogadro number of  atoms or molecules or ions but molar mass is different.