Key concept: molar conductivity of weak electrolytes

Molar conductivity is defined as: $\Lambda_m = \frac{\kappa \times 1000}{C}$ where $\kappa$ is conductivity and $C$ is concentration.

Behavior of weak electrolytes

Weak electrolytes are only partially dissociated in solution. On dilution, their degree of dissociation increases significantly.

Hence, as concentration decreases, the number of ions increases sharply, and therefore molar conductivity increases sharply.

Interpretation of the plot against $\sqrt{C}$

If concentration increases, then $\sqrt{C}$ also increases. Since weak electrolytes show much lower molar conductivity at higher concentration, the plot shows that:

• with increase in concentration, molar conductivity decreases sharply;
• with dilution, molar conductivity increases sharply.

Check the options

• A: Molar conductivity increases sharply with increase in concentration.
  ❌ Incorrect. It actually decreases with increase in concentration.

• B: Molar conductivity decreases sharply with increase in concentration.
  ✅ Correct.

• C: A small increase in molar conductivity is observed at infinite dilution.
  ❌ Incorrect. For weak electrolytes, the increase on dilution is large, not small.

• D: A small decrease in molar conductivity is observed at infinite dilution.
  ❌ Incorrect.

Final answer

Therefore, the correct option is: $\boxed{\text{B}}$

Comparison with stored correct answer

Stored correct answer = B
Derived answer = B
Hence, they agree.