The number 6.02 x 1023 is called Avogadro’s number.
The mass, in grams of 6.02 x 1023 molecules of any substance is equal to its molar mass. In other words, molecules mass of any substance has 6.02 x 1023 molecules.
For example, molecular mass of water H2O = 2 + 16 = 18
.'. Molar mass of H2O = 18 g
.'. In 18 g of H2O molecules,
6.022 x 1023 molecules are there.