One mole of iron III sulfate, #Fe_2(SO_4)_3#, contains how many moles of #SO_4^-2# ions? How many moles of O atoms?
Here, we use the mole as we would use any other collective number: a dozen eggs; a Bakers' dozen; a Botany Bay dozen.
Of course, the mole specifies a much larger quantity, and if I have a mole of stuff then I have
Here we have the formula unit