Mr(CaCO3) = 40 + 12 + 16*3 = 100
W(O) = Ar(O)*n / Mr(CaCO3) *100% = 16*3 / 100 *100% = 48%
Mr(FeCO3) = 56 + 12 + 16*3 = 116
W(O) = Ar(O) *n / Mr(FeCO3) *100% = 16*3 / 116 *100% = 41%
Mr(Zn(NO3)2 = 65 + 14*2 + 16*6 = 189
W(O) = Ar(O) * n / Mr(Zn(NO3)2) *100% = 16*6 / 189 *100% =51%