Chemistry · Electrochemistry · NEET
In a strong electrolyte (like KCl, NaCl, BaCl₂) the substance is fully ionised at all concentrations. So the number of ions does not change much with dilution. The only reason Λm rises a little on dilution is that ions get farther apart, so they slow each other down less (less ion-ion attraction). This gives a small, steady increase, which is why the plot is a straight line that follows Λm = Λ°m − A√c. You can just extend (extrapolate) this straight line back to √c = 0 to read off Λ°m directly.
A weak electrolyte (like acetic acid CH₃COOH, or NH₄OH) is only partly ionised. At normal concentration very few molecules break into ions, so Λm is low. As you dilute a lot, the degree of dissociation (α) jumps up a lot, so suddenly there are many more ions and Λm shoots up steeply. Because this rise is due to more ions forming (not just ions moving apart), the curve keeps climbing sharply near √c = 0 instead of flattening. That is why it is a curve, not a straight line.
No. This is the Debye-Hückel-Onsager equation and it only works for strong electrolytes, where the plot is linear. For weak electrolytes the plot is a steep curve, so a single straight-line equation cannot describe it. This is a very common NEET trap: the statement 'this equation applies to both strong and weak electrolytes' is FALSE.
For a strong electrolyte the line is straight, so extending it back to √c = 0 gives Λ°m easily. For a weak electrolyte the curve rises almost vertically near √c = 0, so there is no straight part to extend — extrapolation gives a wrong answer. Instead, we find Λ°m of a weak electrolyte indirectly using Kohlrausch's law (adding up limiting values of strong electrolytes). This is exactly why the next topic, Kohlrausch's law, exists.
Both. The slope A depends on the nature of the solvent (its viscosity and dielectric constant) AND on the charge type of the electrolyte. A 1:1 electrolyte (like KCl) has a different A than a 2:2 electrolyte (like MgSO₄). But two electrolytes of the SAME charge type have the same A — for example BaCl₂ (2:1) and Mg(OH)₂ (2:1) share the same slope, while BaCl₂ (2:1) and MgSO₄ (2:2) do not.
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.
For a strong electrolyte salt XY, the plot of Λm versus √c has slope −90.0 S cm² mol⁻³ᐟ² L¹ᐟ² at 298 K. At 0.01 M, Λm = 145.0 S cm² mol⁻¹. The limiting molar conductivity of Y⁻ ion, λ°(Y⁻) (in S cm² mol⁻¹), is: [Given λ°(X⁺) = 74.0 S cm² mol⁻¹]
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Strong electrolytes give a nearly straight line with a small downward slope. Weak electrolytes give a curve that rises steeply near √c = 0 (very low concentration) and does not become linear.
Strong electrolyte: KCl, NaCl, HCl, BaCl₂. Weak electrolyte: acetic acid (CH₃COOH), NH₄OH. Strong ones are fully ionised; weak ones are only partly ionised.
Because at high concentration a weak electrolyte is only slightly dissociated (small α), so there are very few ions to carry current. On dilution α increases, more ions form, and Λm rises sharply.
We use Kohlrausch's law of independent migration of ions. We add and subtract the limiting molar conductivities of suitable strong electrolytes to build up the value for the weak electrolyte, for example Λ°m(CH₃COOH) = Λ°m(CH₃COONa) + Λ°m(HCl) − Λ°m(NaCl).
No. A is the same only for electrolytes of the same charge type and the same solvent. A 1:1 salt and a 2:2 salt have different slopes; a 2:1 salt like BaCl₂ has the same slope as another 2:1 salt like Mg(OH)₂.
