Molecular mass of an substance (element or compound) is the average relative mass of a molecule of the substance as compared with mass of an atom of carbon(C-12) taken as 12u.It tell us the number of times a molecule of the substance is heavier than 1/12th of mass of a carbon(C-12) atom.For ex-molecular mass of O2 molecule will be 32.Thus a molecule of O2 is 32 times heavier than the 1/12th of mass of C-12.
On the other hand Avogadro’s no. = 6.023 * 1023 is the amount of any specified elementary partices and is called one mole.Mole is simply a unit for counting atoms,molecules,ions etc.Thus molecular mass of substance is determined relative
to the mass of carbon-atom C-12 and will not change if Avogadro’s no. is taken as any other no.
