Question

Compounds having common ion but different solubility constants can be separated by ------

• A. Post precipitation
• B. Surface attraction
• C. Fractional precipitation
• D. Inclusion

Hint:

"Fractional" implies separating a mixture into distinct fractions or stages.
Whenever you need to separate ions based on differing solubility product constants (\(K_{sp}\)) using a common precipitating ion, the method is always Fractional Precipitation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Precipitation chemistry relies on the solubility product constant (\(K_{sp}\)) of compounds.
When a common precipitating reagent is added to a solution containing multiple ions, the compound with the lower solubility product constant precipitates first.

Step 2: Detailed Explanation:
Fractional precipitation is an analytical separation technique used to separate two or more ions in a single solution by adding a common precipitating reagent.
This separation is possible when the ions form compounds with the common reagent that have significantly different solubility product constants (\(K_{sp}\)).
The compound with the lower \(K_{sp}\) requires a lower concentration of the common ion to exceed its solubility limit, causing it to precipitate first.
For example, if a solution contains both Chloride (\(\text{Cl}^-\)) and Iodide (\(\text{I}^-\)) ions, adding silver nitrate (\(\text{AgNO}_3\)) will cause Silver Iodide (\(\text{AgI}\)) to precipitate first because its solubility product is much lower than that of Silver Chloride (\(\text{AgCl}\)):
\[ K_{sp}(\text{AgI}) \ll K_{sp}(\text{AgCl}) \]
This allows the two ions to be separated sequentially by filtration.
In contrast:
- Post precipitation is an undesirable analytical phenomenon where an impurity precipitates slowly on the surface of the primary precipitate after its formation.
- Inclusion is a coprecipitation defect where impurity ions are trapped inside the crystal lattice of the precipitate.

Step 3: Final Answer:
Compounds can be separated by Fractional precipitation.