Molar mass is the mass of one stable molecule of a compound, that cannot join with another molecule of the same kind to form a new one, while an empirical formula is the molecular formula in its simplest form. E.g. empirical formula of ethene is CH2while its molecular formula is C2H4.
What about it do you want answered
So potassium is more reactive than lithium because the outer electron of a potassium atom is further from its nucleus than the outer electron of a lithium atom. Hope this answers the question. Have a nice day. Feel free to ask more questions.