Read the passage carefully and answer the questions.
The resistance (R) of an object is given by the expression \( ρ\frac{l}{A} \). The inverse of R is called conductance and the inverse of \( ρ \) is called conductivity or specific conductance represented by K. The conductivity depends upon the nature of the material (insulator, semiconductor or conductor) and the temperature of the substance. Similarly, the conductance due to the ions present in the solution (ionic conductance) depends upon the nature of the electrolyte, size of the ions produced, the nature of the solvent and the temperature. Molar conductivity (\( Λ_m \)) of a solution at a given concentration is the conductance of the volume V of solution containing one mole of electrolyte kept between two electrodes with area of cross-section A and the distance of unit length. It is given by the expression, \( Λ_m = kV \). Molar conductivity increases with the decrease in concentration and when the concentration approaches zero, it is called limiting molar conductivity. The decrease is linear in the case of a strong electrolyte. However, in the case of a weak electrolyte, the change in molar conductivity is gradual in the higher concentration range and there is a sharp increase in molar conductivity as the concentration approaches zero. From the limiting molar conductivity values of a strong electrolyte, we can find the limiting molar conductivity of a weak electrolyte containing the common ions using the Kohlrausch law of independent migration of ions. From molar conductivity and limiting molar conductivity, we can Am determine the degree of dissociation (\( \frac{Λ_m}{Λ_m^o} \)) and ionization constant of a weak acid.
Select the correct statement.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Molar conductivity increases with the decrease in concentration for both strong and weak electrolytes.
Two quantities change in opposite directions on dilution. Conductivity (κ) decreases, because the number of ions per unit volume falls. Molar conductivity, \( \Lambda_m = \kappa V \), refers to the volume V that contains one mole of the electrolyte. This volume increases on dilution more than κ decreases, so molar conductivity increases with dilution for every electrolyte. The two kinds of electrolyte differ only in how fast it increases:
Strong electrolytes (such as KCl) are fully ionised at all concentrations. On dilution the ions move farther apart and the interionic attractions weaken, so \( \Lambda_m \) increases slowly, nearly linearly with \( \sqrt{c} \):
\( \displaystyle \Lambda_m = \Lambda^{\circ}_m - A\sqrt{c} \)
Weak electrolytes (such as acetic acid) are only partly ionised. Dilution increases their degree of dissociation, and so the number of ions from one mole, so \( \Lambda_m \) increases steeply, especially near zero concentration.
Why the other options are incorrect
Option (2) Molar conductivity decreases with the decrease in concentration for both strong and weak electrolytes. — Incorrect. This is true of conductivity (κ), not of molar conductivity.
Option (3) Molar conductivity decreases with the decrease in concentration for strong electrolytes only. — Incorrect. The molar conductivity of a strong electrolyte also increases on dilution, though slowly.
Option (4) Molar conductivity decreases with the decrease in concentration for weak electrolytes only. — Incorrect. For a weak electrolyte the increase is in fact the steepest, because more molecules ionise on dilution.
NCERT (2026-27 edition):
Variation of Conductivity and Molar Conductivity with Concentration – “… Molar conductivity increases with decrease in concentration. This is because the total volume, V, of solution containing one mole of electrolyte also increases. It has been found that decrease in κ on dilution of a solution is more than compensated by increase in its volume.”