Read the passage carefully and answer.
The molar conductivity of a solution at any given concentration is the conductance of the volume of solution containing one mole of electrolyte kept between two platinum electrodes with unit area of cross-section and at a distance of unit length. Both conductivity and molar conductivity change with the concentration of the electrolyte. Kohlrausch examined \( Ʌ°_m \) values for a number of strong electrolytes and observed certain regularities. He noted that the difference in \( Ʌ°_m \) of the electrolytes NaX and KX for any X is nearly constant. On the basis of the above observations, he enunciated the Kohlrausch law of independent migration of ions.
Identify electrolyte A and B in the given image.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A is a weak electrolyte, B is a strong electrolyte.
Strong and weak electrolytes behave differently on dilution. A strong electrolyte is already fully dissociated, so \( \Lambda_m \) rises only slowly on dilution, and the plot of \( \Lambda_m \) against \( c^{1/2} \) is a straight line:
\( \displaystyle \Lambda_m = \Lambda^{\circ}_m - A\,c^{1/2} \)
A weak electrolyte dissociates more and more as it is diluted, so \( \Lambda_m \) is small at higher concentration and rises steeply near zero concentration, giving a curve that hugs the axis.
In the figure, curve A shoots up near \( c^{1/2} = 0 \), so A is a weak electrolyte (like acetic acid). Line B is a gently sloping straight line, so B is a strong electrolyte (like KCl). So A is weak and B is strong.
Why the other options are incorrect
Option (1) A is a strong electrolyte, B is weak electrolyte. — Incorrect. This swaps the two: the straight line belongs to the strong electrolyte.
Option (3) Both A and B are strong electrolytes with B having low solubility. — Incorrect. A strong electrolyte never gives the steep rise of curve A, and solubility does not shape this plot.
Option (4) Both A and B are weak electrolytes. — Incorrect. A weak electrolyte does not give a straight line like B.
NCERT (2026-27 edition):
Variation of Conductivity and Molar Conductivity with Concentration – “… For strong electrolytes, Λm increases slowly with dilution and can be represented by the equation: Λm = Λ°m – A c½ … It can be seen that if we plot (Fig. 2.6) Λm against c1/2, 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 Λm increases steeply (Fig. 2.6) on dilution, especially near lower concentrations.”
