CBSE 2025, Set 3

Question

Based on the data given below, give plausible reason for the variation of conductivity and molar conductivity with concentration:

| Concentration of KCl (mol/L) | Conductivity at 298.15 K (S cm$^{-1}$) | Molar Conductivity at 298.15 K (S cm$^2$ mol$^{-1}$) |
|---|---|---|
| 1.000 | 0.1113 | 111.3 |
| 0.100 | 0.0129 | 129.0 |
| 0.010 | 0.00141 | 141.0 |

Edvaya · Accepted Answer

The correct answer is Subjective. **Conductivity decreases with decrease in concentration:** As concentration decreases, the **number of ions per unit volume decreases**. Since conductivity depends directly on the number of current-carrying ions per unit volume, conductivity decreases with dilution. (From 0.1113 → 0.0129 → 0.00141 S cm$^{-1}$) **Molar conductivity increases with decrease in concentration:** Molar conductivity ($\Lambda_m = \kappa 	imes 1000/C$) increases on dilution because: - **Inter-ionic attractions decrease** (ions are farther apart at lower concentration → less retardation of ion movement) - For weak electrolytes: **degree of dissociation increases** with dilution, producing more ions - For strong electrolytes (KCl): reduced inter-ionic interactions allow faster ionic migration (From 111.3 → 129.0 → 141.0 S cm$^2$ mol$^{-1}$)

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