Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2·6H2O. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a common laboratory reagent. Like the other ferrous sulfate salts, ferrous ammonium sulfate dissolves in water to give the aquo complex [Fe(H2O)6]2+, which has octahedral molecular geometry. Ferrous ammonium sulfate hexahydrate is a hydrate that is the hexahydrate form of ferrous ammonium sulfate. Acts as an iron ion donor for building Fe-S clusters in vitro. It is a hydrate and an iron molecular entity.