1) Cr(3)O(2), Cr(6)O(2), Mn(7)O(2), Na(1)O(2)
2) 2H2+O2=2H2O
4P+5O2=2P2O5
2Mg+O2=2MgO
3) Mr(CaO)=40+16=56
Mr(SO2)=32+16*2=64
Mr(K2O)=39*2+16=94
Mr(Mn2O7)=55*2+16*7=222
4) Mr(Cu2O)=64*2+16=144
W(O)=Ar(O)*n/Mr(Cu2O) *100%=16*1/144 *100%=11%
Mr(CuO)=64+16=80
W(O)=Ar(O)*n/Mr(CuO) *100%=16*1/80 *100%=20%