How does the molar conductivity of a weak electrolyte such as acetic…

The correct answer is Option (1) → It increases, very steeply in very dilute solutions

Molar conductivity, Λ_m, is the conductance of the volume of solution that contains 1 mol of the electrolyte. Acetic acid is only partly dissociated; on dilution its degree of dissociation increases, so the same 1 mol gives more ions and Λ_m rises, very steeply at low concentrations.

Why the other options are incorrect

Option (2) It decreases steadily — Incorrect. It is the conductivity (κ) that decreases on dilution, because there are fewer ions per unit volume; molar conductivity increases.

Option (3) It remains constant — Incorrect. The number of ions from 1 mol of a weak electrolyte changes with dilution, so Λ_m cannot stay constant.

Option (4) It first increases and then decreases — Incorrect. Λ_m keeps rising as the concentration falls and reaches its limiting value Λ°_m at infinite dilution, where dissociation is complete; it does not fall.

NCERT (2026-27 edition):
Weak Electrolytes – “Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations and hence for such electrolytes, the change in Λ_m with dilution is due to increase in the degree of dissociation and consequently the number of ions in total volume of solution that contains 1 mol of electrolyte. In such cases Λ_m increases steeply (Fig. 2.6) on dilution, especially near lower concentrations.”

Fig. 2.6: Molar conductivity versus c½ for acetic acid (weak electrolyte) and potassium chloride (strong electrolyte) in aqueous solutions.