12 г                                                 Х г

    2Mg             +           O2        =      2MgO

n = 2 моль                                        n = 2 моль

Mr = 24 г/моль                                 Mr = 40 г/моль

m = 48 г                                            m = 80 г

12 г Mg     -      Х г MgO

48 г Mg     -     80 г MgO

m(MgO) = 12* 80 / 48 = 20 г