72 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

72 г Mg     -     Х г MgO

48 г Mg    -      80 г MgO

m(MgO) = 72 * 80 / 48 =120 г