Which of the following are the example of double salt? (A) FeSO _ 4...

A double salt is a salt that contains two or more different cations or anions.

(A) ${FeSO}_{4} + {\left({NH}_{4}\right)}_{2}{SO}_{4} + 6{H}_{2}O \to {FeSO}_{4} \cdot {\left({NH}_{4}\right)}_{2}{SO}_{4} \cdot 6{H}_{2}O$(Mohr's salt)

Mohr’s salt called double salt which contains more than one simple salt. It undergoes complete dissociation in aqueous solutions.

(B) ${CuSO}_{4} \cdot 4{NH}_{3} \cdot {H}_{2}O$

Ammonia solution in water gives a blue precipitate when it combines with a solution of copper salt. The pale blue precipitate of copper hydroxide dissolves in excess of ammonium hydroxide forming tetraamine copper[II] sulphate, an azure blue (deep blue) soluble complex salt.

(C) ${K}_{2}{SO}_{4} \cdot {Al}_{2}{\left({SO}_{4}\right)}_{3} \cdot 24{H}_{2}O$ (Potash alum)

It is a double salt. Basically, it is comprised of two simple salts potassium sulphate and aluminium sulphate. Potash alum will not be wet in its solid form because the water of crystallisation is not free.

(D) $Fe ( CN{)}_{2} \cdot 4KCN$

It is complex salt. It does not dissociate completely into individual ions.

So, the correct answer is A and C only.