(1) The average mass of a molecule of a substance in the atomic mass unit is called molecular mass, whereas, the mass of 1 mole or 6.02 x 1023 molecules of the substance (in grams) is called its molar mass. Both these terms are numerically the same but differ in units. Molecular mass is expressed in the atomic mass unit (u), whereas, molar mass is expressed in grams.
(2) The mass of 6.02 x 1023 atoms, in grams is equal to its atomic mass. In other words, the atomic mass of every element contains 6.02 x 1023 atoms of that element. For example, Atomic mass of magnesium is 24. So 24-gram magnesium has 6.02 x 1023 atoms of Mg, and a mass of 6.02 x 1023 atoms of magnesium is 24 gram.
