The figure shows how the molar conductivity (Λm) of aqueous solutions…

The correct answer is Option (4) → KCl, because its plot is a straight line that can be extended to c^1/2 = 0

Λ°_m is the molar conductivity when the concentration approaches zero, so it can be read from a plot only when the plot can be extended to c^1/2 = 0.

• KCl (a strong electrolyte) gives a straight line: its Λ_m increases slowly with dilution. The line is extended to c^1/2 = 0, and its intercept gives Λ°_m.

      \( \Lambda_m = \Lambda_m^{\circ} - A\,c^{1/2} \)

• CH₃COOH (a weak electrolyte) gives a curve that rises steeply near low concentrations, as its degree of dissociation increases on dilution. This curve cannot be extended to zero concentration, so its Λ°_m is found by Kohlrausch law instead.

Why the other options are incorrect

Option (1) CH₃COOH, because its Λ_m reaches the highest values near zero concentration — Incorrect. The high values come from the steep rise near zero concentration, and this steep rise is the reason the curve cannot be extrapolated.

Option (2) Both CH₃COOH and KCl, because Λ_m of both increases on dilution — Incorrect. Λ_m of both increases on dilution, but only the KCl plot is a straight line that can be extended; the CH₃COOH curve cannot.

Option (3) Neither of them, because Λ_m of both changes with concentration — Incorrect. A change with concentration does not prevent extrapolation: Λ_m of KCl changes in a straight line with c^1/2, and the intercept of this line gives Λ°_m.

NCERT (2026-27 edition):
Strong Electrolytes – “For strong electrolytes, Λ_m increases slowly with dilution and can be represented by the equation:”

    \( \displaystyle \Lambda_m = \Lambda_m^{\circ} - A\,c^{1/2} \qquad (2.23) \)

“It can be seen that if we plot (Fig. 2.6) Λ_m against c^1/2, we obtain a straight line with intercept equal to Λ°_m and slope equal to ‘–A’.”

Weak Electrolytes – “Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations… In such cases Λ_m increases steeply (Fig. 2.6) on dilution, especially near lower concentrations. Therefore, Λ°_m cannot be obtained by extrapolation of Λ_m to zero concentration.”

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