Which of the following statements are true with reference to the variation of conductivity and molar conductivity as concentration changes?
A. Conductivity decreases with decrease in concentration for weak electrolytes.
B. Conductivity decreases with decrease in concentration for strong electrolytes.
C. Molar conductivity increases slowly with dilution for weak electrolyte.
D. Molar conductivity increases steeply with dilution for strong electrolyte.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and B only
Conductivity (κ) is the conductance of 1 cm3 of solution, so it depends on the number of ions per unit volume. Molar conductivity (Λm) is the conductance of all the ions from one mole of electrolyte:
\( \displaystyle \Lambda_m=\frac{κ}{c} \)
On dilution, there are fewer ions per cm3, so κ falls for every electrolyte. But Λm rises for every electrolyte, in two different ways:
Strong electrolyte: it is already fully ionised. Λm rises only because the ions attract each other less, so the rise is slow and gradual (Λm = Λm° − A√c).
Weak electrolyte: its degree of dissociation rises sharply on dilution, so Λm rises steeply, especially at low concentration.
A. Conductivity decreases with decrease in concentration for weak electrolytes. — Correct. Fewer ions per unit volume means lower conductivity.
B. Conductivity decreases with decrease in concentration for strong electrolytes. — Correct. The same reason applies to strong electrolytes.
C. Molar conductivity increases slowly with dilution for weak electrolyte. — Incorrect. For a weak electrolyte the increase is steep, because more of it dissociates on dilution.
D. Molar conductivity increases steeply with dilution for strong electrolyte. — Incorrect. For a strong electrolyte the increase is slow, because it is already fully ionised.
NCERT (2026-27 edition):
Variation of Conductivity and Molar Conductivity with Concentration
Summary – “… Conductivity decreases but molar conductivity increases with decrease in concentration. It increases slowly with decrease in concentration for strong electrolytes while the increase is very steep for weak electrolytes in very dilute solutions.”