Hydrogen gas and oxygen gas combine to form water. how many grams of oxygen are needed to react with 45.7 grams of hydrogen ?

The balanced equation:
         2 H₂   +   O₂   →   2 H₂O

moles of Hydrogen  = mass of hydrogen ÷ molar mass of hydrogen
                                  =  45.7 g ÷ 2 g/mol
                                  = 22.85 mol

Now, the mole ratio of O₂  :  H₂  is  1  :  2 
    which means that there are two times as much moles of H₂ as O₂ taking
    part in the reaction

Thus since moles of H₂  =  22.85 mol
then moles of O₂  =  22.85 ÷ 2 = 11.425 mol

Mass of O₂  = moles of O₂  × molar mass of O₂

                    = 11.425 mol  ×  32 g/mol

                    =  365.6 g

∴  grams of oxygen are needed to react with 45.7 grams of hydrogen is 365.6g