a) 2Cr+3H2SO4--->Cr2(SO4)3+3H2 siarczan(VI)chromu(III)
2Cr +6H+ +3SO4 2- --->2Cr3+ +3SO4 2- +3H2
2Cr +6H+ --->2Cr3+ +3H2
b) Ba(NO3)2+K2SO4--->BaSO4↓+2KNO3 siarczan(VI)baru i azotan(V)potasu
Ba2+ +2NO3- +2K+ +SO4 2- --->BaSO4↓ +2K+ +2NO3-
Ba2+ +SO4 2- --->BaSO4↓
c) MgO+SO2--->MgSO3 siarczan(IV)magnezu
MgO +SO2 --->Mg2+ +SO3 2-
d) FeO+2HCl--->FeCl2+H2O chlorek żelaza(II)
FeO +2H+ +2Cl- --->Fe2+ +2Cl- +H2O
FeO +2H+ --->Fe2+ +H2O