In the combustion of hydrogen gas, hydrogen reacts with oxygen from the air to form water vapor, if you burn 50.0g of hydrogen and produce 447g of water how much oxygen is reacted

2H₂ ₍g₎  +  O₂ ₍g₎  -------->  2H₂O ₍l₎

If mass of water =  447 g 
And mol =  

then mol of H₂O  =   
                           =  24.83 mol

Mole ratio of  H₂O : O₂    ≡    2  :  1

Since mole of H₂O =  24.83 mol
then mole of O₂  =  24.83 mol  ÷  2
                           =  12.42 mol

Thus Mass of O₂  = 12.42 mol  *  (16 * 2) g / mol
                             = 397.28 g

Answer is to avoid experimental error