The number of correct statements from the following is _______.
(A) Conductivity always decreases with decrease in concentration for both strong and weak electrolytes.
(B) The number of ions per unit volume that carry current in a solution increases on dilution.
(C) Molar conductivity increases with decrease in concentration
(D) The variation in molar conductivity is different for strong and weak electrolytes
(E) For weak electrolytes, the change in molar conductivity with dilution is due to decrease in degree of dissociation.

Solution

Chemistry ma’am · 3 steps

Keep conductivity and molar conductivity apart: on dilution one falls and the other rises.

1. Conductivity. Conductivity κ is the conductance of 1 cm³ of solution. On dilution there are fewer ions in each unit volume, so κ falls for both strong and weak electrolytes.
2. Molar conductivity. Molar conductivity counts the ions from one mole of electrolyte, whatever volume they spread through. On dilution that volume grows faster than κ falls, so Λₘ rises. A strong electrolyte rises slowly as interionic attraction weakens. A weak electrolyte rises sharply because its degree of dissociation α increases.

   Λₘ = (κ × 1000)/c

   Statement	Verdict	Reason
   (A)	Correct	Fewer ions per unit volume, so κ falls
   (B)	Wrong	Ions per unit volume decrease on dilution
   (C)	Correct	Λₘ rises as c falls
   (D)	Correct	Strong: slow, almost linear rise. Weak: steep rise at low c
   (E)	Wrong	α increases on dilution, it does not decrease

3. So. Statements (A), (C) and (D) are correct, so the answer is 3.

The common slip: 4. Counting (E) as correct gives 4. Dilution increases the degree of dissociation of a weak electrolyte, so (E) is false.

The idea to keep. Dilution means fewer ions per cm³, so κ falls. Each mole of electrolyte gives more ions that move freely, so Λₘ rises. For weak electrolytes the cause is a larger α.

Answer from the NTA final answer key. Solution by PrepWiser.