Chemistry · Electrochemistry · NEET
Conductivity κ measures how well 1 cm³ of the solution carries current. It depends on how many ions sit inside that 1 cm³. When you add water, the same ions spread into a bigger volume, so each cm³ now holds FEWER ions. Fewer ions per cm³ means less current per cm³, so κ drops. Nothing was removed, the ions just got more spread out.
Molar conductivity Λm is the conductance of ALL the ions produced by exactly 1 mole of electrolyte, no matter how much water surrounds them. The formula is Λm = κ × 1000 / c. When you dilute, κ drops a little, but the concentration c drops much MORE. Dividing by a much smaller c makes Λm go up. In simple words: κ counts ions in a fixed volume; Λm counts ions from a fixed amount of electrolyte.
It is not a contradiction because they measure two different things. Picture 1 mole of salt. In a concentrated beaker the ions are crammed into a small volume: high κ, but they are counted in a tiny volume. Add water and the same 1 mole of ions spread into a huge volume: κ per cm³ falls, but Λm (which follows the whole mole) rises. Both statements are true at the same time.
For a WEAK electrolyte (like acetic acid), dilution increases the degree of dissociation α, so more molecules break into ions, giving a big rise in Λm. For a STRONG electrolyte (fully ionised already), dilution mainly reduces inter-ionic attraction (ions get farther apart and slow each other down less), giving only a small rise in Λm. This is why the graphs look very different, which the next concept covers.
This is the Debye-Huckel-Onsager equation. Λ°m is the limiting molar conductivity (value at infinite dilution, c → 0). As concentration c increases, the term A√c grows, so Λm drops below Λ°m. It gives a STRAIGHT line only for strong electrolytes when you plot Λm against √c. The slope A depends on the solvent and on the charge type of the electrolyte.
For the electrolyte's contribution, κ keeps falling toward the pure-water value as you keep diluting, because ions per cm³ keep dropping. But Λm reaches a fixed maximum called Λ°m (limiting molar conductivity). So remember: κ → very small on dilution, Λm → maximum (Λ°m).
Molar conductance of an electrolyte increases with dilution according to the equation: Λm = Λ°m − A√c. Which of the following statements are true? (A) This equation applies to both strong and weak electrolytes. (B) Value of the constant A depends upon the nature of the solvent. (C) Value of constant A is same for both BaCl₂ and MgSO₄. (D) Value of constant A is same for both BaCl₂ and Mg(OH)₂.
The molar conductivity of a 0.5 mol dm⁻³ solution of AgNO₃ with electrolytic conductivity of 5.76 × 10⁻³ S cm⁻¹ at 298 K is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Conductivity κ decreases; molar conductivity Λm increases. This is a very common one-mark NEET fact.
The 1000 converts concentration from mol per litre into mol per cm³, because κ is in S cm⁻¹ and volume is measured in cm³. It keeps the units of Λm as S cm² mol⁻¹.
Weak electrolytes show a much bigger rise because dilution increases dissociation (α). Strong electrolytes rise only slightly because they are already fully ionised.
It is the value of Λm at infinite dilution (c → 0), the maximum Λm can reach. It is found by extrapolating the Λm vs √c graph back to the y-axis for strong electrolytes.
Yes. NEET repeatedly asks (1) the direction of change of κ and Λm on dilution, (2) the Λm = Λ°m − A√c statement question, and (3) direct Λm = κ × 1000 / c calculations.
