Question

To a solution of colourless sodium salt, a solution of lead nitrate was added to have a white precipitate which dissolves in warm water and reprecipitates on cooling. Which of the following acid radical is present in the salt?

• A. CI-
• B. SO2-4
• C. S2-
• D. NO-3

Answer 1

The Correct Option is A

The presence of Cl^- (chloride) is indicated by the observations.

Here's a breakdown:

• Reaction with Lead Nitrate: The addition of lead nitrate (Pb(NO₃)₂) to a chloride salt will produce a white precipitate of lead(II) chloride (PbCl₂): $Pb^{2+}(aq) + 2Cl^{-}(aq) \rightarrow PbCl_2(s)$
• Solubility in Warm Water: Lead(II) chloride (PbCl₂) is unique in that it is soluble in hot water. The elevated temperature increases its solubility, causing the precipitate to dissolve: $PbCl_2(s) \xrightarrow{warm \, water} Pb^{2+}(aq) + 2Cl^{-}(aq)$
• Reprecipitation on Cooling: As the warm water cools, the solubility of lead(II) chloride decreases, causing it to reprecipitate out of solution as a white solid.
• Other Anions:
  • Lead sulfate (PbSO₄) is also a white precipitate, but it is largely insoluble in water, even when heated.
  • Lead sulfide (PbS) is a black precipitate.
  • Lead nitrate (Pb(NO₃)₂) is soluble, so nitrate (NO₃^-) would not form a precipitate.

Therefore, the given observations strongly suggest the presence of chloride (Cl^-) ions in the original sodium salt.

Correct Answer: Cl^-