Х г                                          700 г

   SiO2        +          2Mg      =       2MgO       +      Si

                           n = 2 моль                               n = 1 моль

                         Mr = 24 г/моль                           Mr = 28 г/моль

                          m = 48 г                                     m = 28 г

Х г Mg    -    700 г Si

48 г Mg  -    28 г Si

m(Mg) = 48 * 700 / 28 = 1200 г