The Question is from neet-chemistry organic-chemistry-basics qualitative-and-quantitative-analysis, and it was created by the author THIRUPATHI REDDY

Question

A solution containing $Cu^{2+}$ and $Ni^{2+}$ is treated with $H_2S$ in an acidic medium. Which ion will precipitate as a sulfide?

THIRUPATHI REDDY · Accepted Answer

## CORE INFORMATION
✓ **Answer**: A
## CONCEPT FRAMEWORK
 **Primary Concept**: Solubility of metal sulfides in acidic medium
 **Related Topics**: Qualitative analysis, precipitation reactions
 **Prerequisites**: Knowledge of solubility product ($K_{sp}$) and understanding of the common ion effect.
## SOLUTION PATHWAY
**Given Information**:
  A solution containing $Cu^{2+}$ and $Ni^{2+}$ ions.
  Treatment with $H_2S$ in an acidic medium.
**Method & Steps**:
 1. Determine the solubility of CuS and NiS in acidic medium.
  Reasoning: The solubility of metal sulfides depends on the $K_{sp}$ values and the concentration of $S^{2-}$ ions.  In acidic medium, the concentration of $S^{2-}$ ions is low due to the reaction of $H_2S$ with $H^+$ ions: $H_2S ightleftharpoons 2H^+ + S^{2-}$.
  Formula/Rule: $K_{sp}(MS) = [M^{2+}][S^{2-}]$ where M represents the metal cation.
 2. Compare the $K_{sp}$ values of CuS and NiS.
  Working: $K_{sp}(CuS)$ is much smaller than $K_{sp}(NiS)$. This means CuS is much less soluble than NiS. In a low concentration of $S^{2-}$ (acidic medium), only CuS will exceed its solubility product and precipitate.
  Key Point:  Even though both sulfides are insoluble in water, their solubility in acidic solutions differs significantly due to the difference in their $K_{sp}$ values.
 3. Identify the precipitating ion.
  Result: Only $Cu^{2+}$ will precipitate as CuS in an acidic medium.
  Verification: The low $K_{sp}$ of CuS ensures that it will precipitate even at low sulfide ion concentrations, while NiS will remain in solution.
 ## OPTION ANALYSIS
A ✓: $Cu^{2+}$ precipitates as CuS because its solubility product is much lower than that of NiS, making it less soluble in acidic conditions.
B :x: $Ni^{2+}$ does not precipitate as NiS in an acidic medium because its solubility product is relatively higher than that of CuS.
C :x: Only $Cu^{2+}$ precipitates, not $Ni^{2+}$.
D :x: $Cu^{2+}$ does precipitate.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Assuming both ions will precipitate:  Students might overlook the difference in $K_{sp}$ values and the effect of acidity on sulfide ion concentration.
  2.Incorrectly predicting NiS precipitation:  Failing to consider the lower concentration of sulfide ions in an acidic environment can lead to an incorrect prediction.
 **Quick Tips**:
  1.Remember that lower $K_{sp}$ values indicate lower solubility and a higher tendency to precipitate.
  2.Focus on the effect of acidity on the concentration of sulfide ions in the solution.

THIRUPATHI REDDY · Answer

The correct Option is "$Cu^{2+}$"

THIRUPATHI REDDY · Answer

$Ni^{2+}$

THIRUPATHI REDDY · Answer

Both $Cu^{2+}$ and $Ni^{2+}$

THIRUPATHI REDDY · Answer

Neither $Cu^{2+}$ nor $Ni^{2+}$

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