8 г                           Х г

    2H2           +           O2            =         2H2O

                               n = 1 моль              n = 2 моль

                           Mr = 32 г/моль            Mr = 18 г/моль

                          m = 32 г                        m = 36 г

8 г О2   -  Х г Н2О

32 г О2  -  36 г Н2О

m(H2) = 8 * 36 / 32 = 9 г