-`Fe_2O_3`
$M_{Fe_2O_3}$`=112+48=160`g/mol
$%m_{Fe}$=$\dfrac{112.100}{160}$`=70%`
$%m_{O}$`=100%-70%=30%`
-`SO_2`
$M_{SO_2}$`=(16.2)+32=64`g/mol
$%m_{S}$=$\dfrac{32.100}{64}$`=50%`
$%m_{O}$`=100%-50%=50%`
-`CuSO_4`
$M_{CuSO_4}$`=64+32+(16.4)=160`g/mol
$%m_{Cu}$=$\dfrac{64.100}{160}$`=40%`
$%m_{S}$=$\dfrac{32.100}{160}$=`20%`
$%m_{O}$=`100%-(20%+40%)=40%`