Step 1: Understanding the Question:
The question asks about the nature of functional groups present in cation-exchange resins used for water purification.
Ion exchange resins are cross-linked polymers that have fixed charged groups to which ions of opposite charge are bound.
Step 2: Detailed Explanation:
Cation Exchange Resins:
These resins are used to remove cations (positive ions like \(Ca^{2+}, Mg^{2+}, Na^+\)) from water.
To perform this exchange, the resin must contain acidic functional groups that can release hydrogen ions (\(H^+\)) or other cations in exchange for the metallic cations in water.
Functional Groups in Resins:
1. Strong Acid Cation Resins: These typically contain sulfonic acid groups (\(-SO_3H\)). They are highly effective and can work across a wide pH range. The \(H^+\) from the \(-SO_3H\) group is easily swapped for cations in the water.
2. Weak Acid Cation Resins: These usually contain carboxylic acid groups (\(-COOH\)).
3. Anion Exchange Resins: These contain basic groups like amino (\(-NH_2\), \(-NHR\), \(-NR_2\)) or quaternary ammonium groups (\(-NR_3^+OH^-\)). These groups exchange anions (like \(Cl^-\)) for hydroxyl (\(OH^-\)) ions.
Analysis of Options:
- \(-SO_3H\) is the characteristic group of strong acid cation exchange resins.
- \(-OH\) and \(-NH_2\) are associated with anion exchange resins (basic nature).
- \(-CHO\) (aldehyde group) is not an ionizable group used in standard ion-exchange resins for water treatment.
Step 3: Final Answer:
The functional group associated with cation exchange resins is \(-SO_3H\) (sulfonic acid group).