When FeSO4is combined with (NH4)2SO4 in a 1: 1 molar ratio, a double salt is formed. FeSO4 (NH4)2SO4 .6H2O. This salt is in charge of supplying Fe 2+ . CuSO 4 in a 1:4 ratio with aqueous ammonia produces a complex salt. The complex salt fails the test because it does not ionize to produce Cu2+.