Question

A is a colourless substance. The aqueous solution of A gives a reddish-orange ppt with KI; ppt dissolves in excess of KI forming a colourless solution. If NH4​Cl and NaOH solution is added to this colourless solution reddish brown ppt is formed. Substance A is:
a. Epsom salt
b. Mohr's salt
c. Calomel
d. Corrosive sublimate

Answer 1

Hint: The given compound is a white crystalline solid and is a laboratory reagent and a molecular compound that is very toxic to humans. With this property in mind try to figure out what the given substance could possibly be.

Complete step by step answer:
Let us look into each of these compounds one at a time and see which of the chemical substances in the given options fit the bill.
1. Epsom salt is the hepta-hydrate form of Magnesium Sulphate ($MgS{{O}_{4}}.7{{H}_{2}}O$) which is a white colourless solid which is often used in soaps and is referred to as bath salts in regular use.
2. Ammonium iron (II) sulphate, or Mohr's salt, is a light green inorganic compound with the formula \[{{\left( N{{H}_{4}} \right)}_{2}}Fe{{\left( S{{O}_{4}} \right)}_{2}}{{\left( {{H}_{2}}O \right)}_{6}}\] which contains two different cations, \[F{{e}^{2+}}\] and NH4+It is classified as a double salt of ferrous sulphate and ammonium sulphate due to the presence of both cations present with its structure. Used as a common laboratory reagent because it is readily crystallized, unlike the other ferrous sulphate salts, it dissolves in water to give the aquo complex \[{{\left[ Fe{{\left( {{H}_{2}}O \right)}_{6}} \right]}^{2+}}\].
3. Calomel is a mercury chloride mineral with formula \[H{{g}_{2}}C{{l}_{2}}\] which is a white powder when completely pure. However, when it is exposed to light or contains impurities it takes on a darker tint.
4. Mercury(II) chloride or mercuric chloride (also commonly referred to as corrosive sublimate) is the chemical compound of mercury and chlorine which has the chemical formula $HgC{{l}_{2}}$. It is also a white crystalline solid in appearance and is used as a laboratory reagent and is chemically a molecular compound that is very toxic to humans.
Judging from physical appearance, we can eliminate Mohr’s salt as a possible answer due to its greenish colour. Now, from the given reactions, we find that the compound in question is corrosive sublimate as it is the only compound which gives out the required products as specified in the reaction since it is a colourless substance which reacts with KI to form reddish orange ppt

\[HgC{{l}_{2}}+2KI\to Hg{{I}_{2}}{{\downarrow }_{(reddish~orange)}}+2C{{l}^{-}}\]

This ppt dissolves in excess of KI.

\[Hg{{I}_{2}}+2KI\to {{K}_{2}}[Hg{{I}_{4}}]\]
Thus, we can conclude that the answer to this question is d) Corrosive sublimate.

Note: Mercuric chloride is occasionally used to form an amalgam with metals, such as aluminium. Upon treatment with an aqueous solution of mercuric chloride, aluminium strips quickly become covered by a thin layer of the amalgam.