4 моль                                          Х г

     2HgO       =       2Hg          +         O2

n = 2 моль                                     n = 1 моль

                                                     Mr = 32 г/моль

                                                     m = 32 г

4 моль HgO     -     Х г О2

2 моль HgO    -    32 г О2

m(O2) = 4 * 32 / 2 = 64 г