1. Use Kohlrausch’s law for strong electrolytes

For a strong electrolyte like NaCl, the equivalent conductance (or molar/equivalent conductivity) decreases with concentration according to:

where:

• $\lambda_C$ = equivalent conductance at concentration $C$
• $\lambda_\infty$ = equivalent conductance at infinite dilution
• $B > 0$ is a constant

2. Reason for the negative sign

As concentration increases, inter-ionic interactions increase, which reduce the mobility of ions. Therefore conductance at finite concentration is less than conductance at infinite dilution:

So the correction term must be subtracted, not added.

3. Check dependence on concentration

For strong electrolytes, the decrease is proportional to $\sqrt{C}$, not $C$.

Thus the correct relation is:

4. Evaluate options

• A: $\lambda_C = \lambda_\infty + BC$ ❌ wrong sign and wrong dependence
• B: $\lambda_C = \lambda_\infty - BC$ ❌ wrong dependence
• C: $\lambda_C = \lambda_\infty - B\sqrt{C}$ ✅ correct
• D: $\lambda_C = \lambda_\infty + B\sqrt{C}$ ❌ wrong sign

Therefore, the correct option is C.