Question

When KI(excess) is added to:
(i) $CuS{{O}_{4}}$(ii) $HgC{{l}_{2}}$(iii) $Pb{{(N{{O}_{3}})}_{2}}$
(A) a white precipitate of CuI in(i), an orange ppt of $Hg{{I}_{2}}$in (ii), and a yellow ppt of $Pb{{I}_{2}}$ in (iii) are formed.
(B) a white ppt of $C{{u}_{2}}{{I}_{2}}$ in (i), a red ppt dissolving to $HgI_{4}^{2-}$ in (ii), and a yellow ppt. Of $Pb{{I}_{2}}$in (iii) are formed.
(C) a white ppt of CuI, $Hg{{I}_{2}}$ and $Pb{{I}_{2}}$ are formed in each case.
(D) none is correct.

Answer 1

When KI is added in excess to certain compounds, iodide compounds are formed. Each iodide gives a characteristic precipitate by which we can identify the iodide formed.

Complete step by step solution:
KI reacts with copper sulphate and forms a white precipitate of $C{{u}_{2}}{{I}_{2}}$.
$4KI+2CuS{{O}_{4}}\to C{{u}_{2}}{{I}_{2}}\downarrow +{{I}_{2}}+2{{K}_{2}}S{{O}_{4}}$
$HgC{{l}_{2}}$ Reacts with KI and forms an orange precipitate of $Hg{{I}_{2}}$ which dissolves in excess of KI to give Nessler’s solution.
$HgC{{l}_{2}}+2KCl\to Hg{{I}_{2}}\downarrow +2KCl$
Excess of KI the reaction is $Hg{{I}_{2}}+KI\to {{K}_{2}}Hg{{I}_{4}}$
When KI reacts with $Pb{{(N{{O}_{3}})}_{2}}$, yellow precipitate is formed of $Pb{{I}_{2}}$
$2KI+Pb{{(N{{O}_{3}})}_{2}}\ to Pb{{I}_{2}}+2KN{{O}_{3}}$
Thus, when KI is added to $CuS{{O}_{4}}$,$HgC{{l}_{2}}$, and $Pb{{(N{{O}_{3}})}_{2}}$ ,we get a white precipitate of $C{{u}_{2}}{{I}_{2}}$ , an orange ppt of $Hg{{I}_{2}}$, and a yellow ppt of $Pb{{I}_{2}}$ are formed respectively.

The answer to the question is option (B).

Additional information:
KI or potassium iodide has many applications. It is used in quantitative analysis. It can also be used as a medication as well as a dietary supplement. Potassium iodide is a good reducing agent. The iodide ion is easily oxidised to ${{I}_{2}}$. KI is used in redox titrations. Aqueous $K{{I}_{3}}$ is used as a disinfectant. It is a good fluorescence quenching agent in biomedical research. Potassium iodide has its most important application in organic synthesis, mainly in the preparation of aryl iodides and the reaction is referred to as Sandmeyer’s reaction.

Note: We can get confused with the reaction of KI with$HgC{{l}_{2}}$. In this reaction only in excess of KI Nessler’s solution is formed. Otherwise, an orange precipitate is formed.