Ferrous chloride is used in metallurgy as an electrolytic agent and as a mordant in dyeing. It has been used in the synthesis of pentasubstituted acylferrocenes. In biological research, ferrous chloride is used as a source of Fe2+ ion.

Application

ron(II)chloride tetrahydrateisusedas a reducing agent in metallurgy, in pharmaceutical preparations, as a mordant in dyeing and in sewage treatment. ... It is the precursor to hydratediron(III) oxides that are magnetic pigments. 

Technical Specification

FeCl2.4H2O is 198.83 g/mol and FeCl2 is 126.75 g/mol - This means that 1g of FeCl2.4H2O is equivalent to 0.6375g of FeCl2.