I have tried to look it up, and find answers, it really intrigues me!
A mole of any substance is defined as Avogadro's number of molecules of that substance. So every mole has the same number of molecules. Since the masses of molecules differ, the mass of a mole of any substance will be dependent on the mass of one molecule of that substance. That is,
(Mass of one mole) = (Avogadro's number) x (the mass of one molecule).