JEE Challenger
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ChemHardAdv 2021 • Paper 1

Identification of Salt Mixture from Precipitation Reactions

A mixture of two salts is used to prepare an aqueous solution S\mathbf{S}, which yields the following experimental observations:

White precipitate(s) only←Room temperatureDilute NaOH(aq)S (aq solution of the salts)→Room temperatureDilute HCl(aq)White precipitate(s) only\begin{array}{ccc} \text{White precipitate(s) only} & \xleftarrow[\text{Room temperature}]{\text{Dilute NaOH(aq)}} & \mathbf{S} \text{ (aq solution of the salts)} & \xrightarrow[\text{Room temperature}]{\text{Dilute HCl(aq)}} & \text{White precipitate(s) only} \end{array}

Identify the correct option(s) for the constituent salt mixture:

Question Diagram 1

Options

A

Pb(NO3)2\text{Pb(NO}_3)_2 and Zn(NO3)2\text{Zn(NO}_3)_2

Correct
B

Pb(NO3)2\text{Pb(NO}_3)_2 and Bi(NO3)3\text{Bi(NO}_3)_3

Correct
C

AgNO3\text{AgNO}_3 and Bi(NO3)3\text{Bi(NO}_3)_3

D

Pb(NO3)2\text{Pb(NO}_3)_2 and Hg(NO3)2\text{Hg(NO}_3)_2

Topics & Concepts

p-Block Elementsp-Block Elementsd-Block Elements

Step-by-Step Solution

To determine the correct constituent salt mixture(s), we analyze the experimental observations for each metal cation present in the given options:

1. Reaction with Dilute NaOH(aq)\text{NaOH(aq)} at Room Temperature:

  • Lead ion (Pb2+\text{Pb}^{2+}): Pb2+(aq)+2OH−(aq)→Pb(OH)2(s)↓(White precipitate)\text{Pb}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Pb(OH)}_2(\text{s}) \downarrow \quad (\text{White precipitate})

  • Zinc ion (Zn2+\text{Zn}^{2+}): Zn2+(aq)+2OH−(aq)→Zn(OH)2(s)↓(White precipitate)\text{Zn}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Zn(OH)}_2(\text{s}) \downarrow \quad (\text{White precipitate})

  • Bismuth ion (Bi3+\text{Bi}^{3+}): Bi3+(aq)+3OH−(aq)→Bi(OH)3(s)↓(White precipitate)\text{Bi}^{3+}(\text{aq}) + 3\text{OH}^-(\text{aq}) \rightarrow \text{Bi(OH)}_3(\text{s}) \downarrow \quad (\text{White precipitate})

  • Silver ion (Ag+\text{Ag}^+): 2Ag+(aq)+2OH−(aq)→Ag2O(s)↓+H2O(l)(Brown/Black precipitate)2\text{Ag}^+(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Ag}_2\text{O}(\text{s}) \downarrow + \text{H}_2\text{O}(\text{l}) \quad (\text{Brown/Black precipitate}) Since a brown precipitate is formed, any mixture containing AgNO3\text{AgNO}_3 does not yield white precipitate(s) only. Thus, Option (C) is incorrect.

  • Mercury(II) ion (Hg2+\text{Hg}^{2+}): Hg2+(aq)+2OH−(aq)→HgO(s)↓+H2O(l)(Yellow precipitate)\text{Hg}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{HgO}(\text{s}) \downarrow + \text{H}_2\text{O}(\text{l}) \quad (\text{Yellow precipitate}) Since a yellow precipitate is formed, any mixture containing Hg(NO3)2\text{Hg(NO}_3)_2 does not yield white precipitate(s) only. Thus, Option (D) is incorrect.

2. Reaction with Dilute HCl(aq)\text{HCl(aq)} at Room Temperature:

  • For Option (A) [Pb(NO3)2\text{Pb(NO}_3)_2 and Zn(NO3)2\text{Zn(NO}_3)_2]:

    • Pb2+\text{Pb}^{2+} reacts with Cl−\text{Cl}^- to form a white precipitate of lead chloride: Pb2+(aq)+2Cl−(aq)→PbCl2(s)↓(White precipitate)\text{Pb}^{2+}(\text{aq}) + 2\text{Cl}^-(\text{aq}) \rightarrow \text{PbCl}_2(\text{s}) \downarrow \quad (\text{White precipitate})
    • Zn2+\text{Zn}^{2+} ions form soluble ZnCl2(aq)\text{ZnCl}_2(\text{aq}) and remain in solution without forming any precipitate.
    • Thus, the only precipitate formed is white (PbCl2\text{PbCl}_2). Option (A) satisfies the conditions.

  • For Option (B) [Pb(NO3)2\text{Pb(NO}_3)_2 and Bi(NO3)3\text{Bi(NO}_3)_3]:

    • Pb2+\text{Pb}^{2+} reacts to form a white precipitate of PbCl2(s)\text{PbCl}_2(\text{s}).
    • Bi3+\text{Bi}^{3+} hydrolyzes in dilute HCl\text{HCl} to form a white precipitate of bismuth oxychloride (BiOCl\text{BiOCl}): Bi3+(aq)+Cl−(aq)+H2O(l)⇌BiOCl(s)↓+2H+(aq)(White precipitate)\text{Bi}^{3+}(\text{aq}) + \text{Cl}^-(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{BiOCl}(\text{s}) \downarrow + 2\text{H}^+(\text{aq}) \quad (\text{White precipitate})
    • Thus, only white precipitates (PbCl2\text{PbCl}_2 and BiOCl\text{BiOCl}) are formed. Option (B) satisfies the conditions.

Conclusion:

The correct options for the constituent salt mixture are (A) and (B).

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