Which of the following cations (i.e., basic radicals) forms colored (not white) precipitate with aqueous solution of potassium iodide but the precipitate does not dissolve in excess of reagent ?

H g^{2 +}

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H g_{2}^{2 +}

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B i^{3 +}

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C u^{2 +}

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Solution

The correct option is C $H g_{2}^{2 +}$
Option (B) is correct.

(a) $H g^{2 +} + 2 I^{-} ⟶ H g I_{2} ↓ (r e d / s c a r l e t)$
$H g I_{2} + 2 I^{-} ⟶ [H g I_{4}]^{2 -} (s o l u b l e c o m p l e x)$

(b) $H g_{2}^{2 +} + 2 I^{-} ⟶ H g_{2} I_{2} ↓ (g r e e n)$
$H g_{2} I_{2} + 2 I^{-} ⟶ [H g I_{4}]^{2 -} + H g ↓ (b l a c k)$

(c) $B i^{3 +} + 3 I^{-} ⟶ B i I_{3} ↓ (b l a c k)$
$B i I_{3}^{-} ↓ + I^{-} ⟶ [B i I_{4}]^{-} (o r a n g e c o l o r s o l u b l e c o m p l e x)$

(d) $2 C u^{2 +} + 5 I^{-} ⟶ 2 C u I ↓ (w h i t e) + I_{3}^{-}$

Therefore, we can see that in option B, the precipitate is not soluble in excess of the reagent and remain as black Mercury precipitate

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