What is an example for a double salt?
A.Cuprammonium sulphate
B.Mohr’s salt
C.Potassium ferricyanide
D.Hexaminecobalt (III) chloride

Hint: We have to know that the salt is a substance which is produced by an acid – base reaction. A salt contains one positive ion and a negative ion. The positive ion is known as cation and the negative ion is known as anion. The positive ion is for the base and negative ion is for the cation. The sodium chloride is an example of salt. The sodium chloride is an ionic compound having the chemical formula, $NaCl$. And the salts are generally white crystalline in solids which do not have any smell and it is easily soluble in water.

Complete answer:
Cuprammonium sulfate does not contain more than one anion or cation. Hence, option (A) is incorrect.
Double salt is a substance containing more than one anion or more than one cation. And this salt crystallizes as one substance but it will ionize into two different salts. Here, the Mohr’s salt is the example for double salt. The Mohr’s salt is also known as ammonium iron (II) sulfate and the formula is, \[{\left( {N{H_4}} \right)_2}Fe{\left( {S{O_4}} \right)_2}{\left( {{H_2}O} \right)_6}\]. Here, the Mohr’s salt contains two primary cations and that is, ammonium cation, \[NH_4^ + \]and ferrous cation, \[F{e^{2 + }}\]. Therefore, the Mohr’s salt is an example of double salt. Hence, option (B) is correct.
Potassium ferrocyanide is not an example of double salt. Because it contains only one cation, which is potassium ion. Hence, option (C) is incorrect.
The Hexaminecobalt (III) chloride is not an example of double salt. Because, it does not contain more than one cation or anion. Hence, option (D) incorrect.

Hence, option (B) is correct.

Note:
We need to know that a double salt is a salt containing more than one anion or more than one cation. In the case of Mohr’s salt, it contains two cations. One is ammonium cation and the other is iron cation. The ammonium ferrous sulphate is light green crystalline solid and it is mainly used for analytical chemistry. It is a preferred source of ferrous ions because of its resistance to oxidation and solid’s extended shelf life.