Which of the following is an example of a double salt?

The correct answer is option 2. Mohr's salt

Mohr's salt is an example of a double salt. It is also known as ferrous ammonium sulfate \([(NH_4)_2Fe(SO_4)_2·6H_2O]\). Double salts are crystalline compounds that result from the combination of two simple salts in stoichiometric proportions. In the case of Mohr's salt, it is formed by the combination of ferrous sulfate \((FeSO_4)\) and ammonium sulfate \(((NH_4)_2SO_4)\) in a 1:1 molar ratio.

Let us look at the other options

(1) Cuprammonium sulphate - This is not a double salt. It is formed through the dissolution of copper(II) oxide or copper(II) hydroxide in excess ammonia to form a complex ion \([Cu(NH_3)_4]^{2+}\) in the solution.

(3) Potassium ferricyanide - This is not a double salt. It is a coordination compound formed by the coordination of potassium ions and ferricyanide ions \([Fe(CN)_6]^{3-}\).

(4) Cobalthexammine chloride - This is not a double salt. It is a complex compound formed by the coordination of cobalt(III) ions with six ammonia \((NH_3)\) ligands.