Mr(Cr2O3) = 52*2 + 16*3 = 152
W(Cr) = Ar(Cr) *n / Mr(Cr2O3) *100% = 52*2 / 152 *100% =68%
W(O) = Ar(O) *n / Mr(Cr2O3) *100% = 16*3 / 152 *100% = 32%
Mr(ZnCO3) = 65 + 12 + 16*3 = 125
W(Zn) = Ar(Zn) *n / Mr(ZnCO3) *100% = 65*1 / 125 *100% = 52%
W(C) = Ar(C) *n / Mr(ZnCO3) *100% = 12 / 125 *100% = 9,6%
W(O) = Ar(O) *n / Mr(ZnCO3) *100% = 16*3 / 125 *100% =38,4%