Write the major species in the water solution of Mg(NO3)2 ,FeCl3 ?

The major species are ##Mg_((aq))^(2+)##, ##NO_(3(aq))^-##, ##Fe_((aq))^(3+)## and ##Cl_((aq))^-##

If you would like a bit more detail:

##Fe_((aq))^(3+)## is an ##Fe^(3+)## ion surrounded by 6 water molecules:

##[Fe(H_2O)_(6)]^(3+)##

The small, highly charged ##Fe^(3+)## ion weakens the O-H bond causing ##H^+## ions to form making the solution slightly acidic.:

##[Fe(H_2O)_6]^(3+)rightleftharpoons[Fe(H_2O)_5(OH)]^(2+)+H^+##

Most transition metal salts are acidic for this reason.