Given the equation #2H_2O -> 2H_2 + O_2#, how many moles of #H_2O# be required to produce 2.5 moles of #O_2#?
From the equation you'll see that
So every 2 molecules of water gives 1 molecule of oxygen.
This ratio (
So to produce 2.5 moles of oxygen, you will need the double number of moles of water, i.e. 5 moles.