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  4. Gradual addition of potassium iodide solution to aqueous Bi(N O3)3 solution initially produces a dark brown precipitate which dissolves in excess of KI to give a clear yellow solution. Identify the yellow colour solution.

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Q. Gradual addition of potassium iodide solution to aqueous $Bi\left(N O_{3}\right)_{3}$ solution initially produces a dark brown precipitate which dissolves in excess of $KI$ to give a clear yellow solution. Identify the yellow colour solution.

NTA AbhyasNTA Abhyas 2022

Solution:

At first $Bi \left( NO _3\right)_3$ undergoes hydrolysis.
Nitric acid is formed which oxidises $I^{-}$to iodine. The liberated iodine dissolves in $K I$ to form yellow precipitate of $KI _3$.
$Bi \left( NO _3\right)_3+ H _2 O \rightarrow\left[ Bi ( OH )\left( NO _3\right)_2\right]+ HNO _3$
$2 HNO _3 \rightarrow H _2 O +2 NO _2+[ O ]$
$2 KI + H _2 O +[ O ] \rightarrow 2 KOH +\underset{\text { Dark brown }}{ I _2}$
$I_2+K I \rightarrow \underset{\text { yellow solution }}{K I_3}$