Okay Mr (H2O)= 18g therefore moles of H2O is 720.8/18= 40.04mol the ratio of H2 to O2 to H2O is 2 : 1 : 2 so moles of H2 is same as H2O here H2= 40.04moles
moles of O2 is half so 40.04 x 0.5 20.02moles
grams of O2 is its moles into Mr of O2 that's 20.02 x 32 = 640.64g
