Weak electrolytes have
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → lower degree of dissociation at higher concentration.
A weak electrolyte such as acetic acid dissociates only partly in water, and an equilibrium is set up between the undissociated molecules and the ions:
\( \displaystyle \ce{CH3COOH <=> CH3COO^- + H^+} \)
For a concentration C and dissociation constant Ka, the degree of dissociation α is:
\( \displaystyle K_a=\frac{C\alpha^2}{1-\alpha}\approx C\alpha^2 \)
\( \displaystyle \alpha\approx\sqrt{\frac{K_a}{C}} \)
So α falls as C rises: a weak electrolyte has a lower degree of dissociation at higher concentration, and dilution increases α.
Why the other options are incorrect
Option (1) lower degree of dissociation at lower concentration — Incorrect. This is the reverse of the real trend; dilution increases α.
Option (3) no dissociation at all — Incorrect. A weak electrolyte does dissociate, only partly; a substance that gives no ions at all is a non-electrolyte, such as glucose or urea.
Option (4) higher dissociation in presence of common ion — Incorrect. A common ion (\( \ce{CH3COO^{-}} \) from \( \ce{CH3COONa} \), for example) shifts the equilibrium to the left by Le Chatelier’s principle, so it suppresses dissociation. This is the common ion effect.
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 Λm 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. …”