Passing an electric current through a sample of water (h2o) can cause the water to decompose into hydrogen gas (h2) and oxygen gas (o2) according to the following equation.
2h2o mc026-1.jpg 2h2 + o2
the molar mass of h2o is 18.01 g/mol. the molar mass of o2 is 32.00 g/mol. what mass of h2o, in grams, must react to produce 50.00 g of o2?

moles of oxygen in reaction = 
                                     
                                             =  
                                      
                                             = 1.563 mol

Mole Ratio of oxygen to water in the reaction =  1 : 2
    ∴ for every mole of oxygen there is two moles of water

Thus moles of water in the reaction = (moles of oxygen)  *  2
                                                         =  1.563 mol  *  2
                                                         =  3.126 mol

Mass of water needed to produce amount of O = mol * molar mass
                                         = (3.126 mol)  *  (18.01  g/mol)
                                         =  56.299 g
                                         ≈  56. 3 g