Why does specific conductivity decrease with dilution for both strong and weak electrolytes?

Specific conductivity ($kappa$) is a measure of the conductance of a unit volume of the solution. When a solution is diluted, the total number of ions remains the same, but they are spread out over a larger volume. This means that the number of ions present in any given unit volume (e.g., 1 cm$^3$) decreases. Since fewer charge carriers are available in that specific volume to transport electricity, the specific conductivity of the solution decreases. This holds true regardless of whether the electrolyte is strong or weak, as the fundamental principle of reduced ion density per unit volume applies to both.

Why does molar conductivity increase with dilution for strong electrolytes?

For strong electrolytes, which are already fully dissociated, dilution primarily affects the inter-ionic interactions. At higher concentrations, ions are closer, leading to stronger attractive forces between oppositely charged ions. These forces hinder their movement. Upon dilution, ions move further apart, reducing these inter-ionic attractions and increasing their effective mobility. Additionally, the viscous drag on ions decreases in a more dilute solution. The combined effect of increased ionic mobility and reduced hindrance outweighs the decrease in ion density per unit volume, leading to an overall increase in molar conductivity.

Why does molar conductivity increase significantly with dilution for weak electrolytes?

The primary reason for the significant increase in molar conductivity ($Lambda_m$) for weak electrolytes upon dilution is the increase in their degree of dissociation ($alpha$). Weak electrolytes only partially dissociate into ions. According to Ostwald's Dilution Law, as the solution is diluted, the equilibrium shifts to favor more dissociation, producing a greater number of ions. Since molar conductivity is directly proportional to the number of ions produced by one mole of electrolyte, this increase in ion count leads to a sharp rise in $Lambda_m$. Increased ionic mobility due to reduced inter-ionic attractions also plays a role, but the increase in the number of charge carriers is the dominant factor.

What is the significance of molar conductivity at infinite dilution ($Lambda_m^\circ$)?

Molar conductivity at infinite dilution ($Lambda_m^\circ$) represents the maximum possible molar conductivity an electrolyte can achieve. At infinite dilution, inter-ionic attractions become negligible, and ions move completely independently of each other. For strong electrolytes, $Lambda_m^\circ$ can be determined by extrapolating the $Lambda_m$ vs. $sqrt{c}$ plot. For weak electrolytes, it cannot be found by extrapolation but is crucial for calculating the degree of dissociation and dissociation constant using Kohlrausch's Law. It provides a benchmark for the intrinsic conducting ability of an electrolyte's ions.

How is the degree of dissociation ($alpha$) related to molar conductivity for weak electrolytes?

For weak electrolytes, the degree of dissociation ($alpha$) is directly related to its molar conductivity. The ratio of the molar conductivity at a given concentration ($Lambda_m$) to the molar conductivity at infinite dilution ($Lambda_m^\circ$) gives the degree of dissociation. The formula is $\alpha = \frac{\Lambda_m}{\Lambda_m^\circ}$. This relationship is extremely useful because it allows us to calculate how much of a weak electrolyte has dissociated into ions at a particular concentration, provided we know its molar conductivity at that concentration and its molar conductivity at infinite dilution (which can be determined using Kohlrausch's Law).

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Version 1Updated 22 Mar 2026

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The conductivity of an electrolytic solution, which is its ability to conduct electricity, is fundamentally dependent on the concentration of ions present within it. This relationship is not straightforward, as two distinct measures, specific conductivity ( $kappa$ ) and molar conductivity ( $Lambda_m$ ), exhibit contrasting trends with changes in concentration. Specific conductivity, representing the…

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Electrolytic solutions conduct electricity due to the movement of ions. The ability to conduct is quantified by specific conductivity ( $kappa$ ) and molar conductivity ( $Lambda_m$ ). Specific conductivity, the conductance of a unit volume, decreases with dilution for both strong and weak electrolytes because the number of ions per unit volume reduces.

Molar conductivity, the conductance of one mole of electrolyte, increases with dilution for both types. For strong electrolytes, this increase is due to reduced inter-ionic attractions and increased ionic mobility, following the Debye-Hückel-Onsager equation ( $Lambda_m = \Lambda_m^\circ - A\sqrt{c}$ ).

For weak electrolytes, the increase is much steeper and primarily due to an increase in the degree of dissociation ( $alpha$ ) as per Ostwald's Dilution Law, which produces more ions. Molar conductivity at infinite dilution ( $Lambda_m^\circ$ ) is the maximum conductivity, obtainable by extrapolation for strong electrolytes, but requiring Kohlrausch's Law for weak electrolytes.

Understanding these variations is crucial for characterizing electrolytes and solving related numerical problems in NEET.

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Key Concepts

Specific Conductivity (

kappa

) and Dilution

Specific conductivity, $kappa$ , is an intensive property, representing the conductance of a fixed volume of…

Molar Conductivity (

Lambda_m

) for Strong Electrolytes

Molar conductivity, $Lambda_m$ , considers the total conductance of one mole of electrolyte. For strong…

Molar Conductivity (

Lambda_m

) and Degree of Dissociation for Weak Electrolytes

For weak electrolytes, the increase in $Lambda_m$ with dilution is much more pronounced and primarily driven…

Specific Conductivity ($kappa$) — Conductance of unit volume. Unit: S cm

^{-1}

Trend with Dilution —

kappa

decreases for both strong & weak electrolytes (fewer ions per unit volume).

Molar Conductivity ($Lambda_m$) — Conductance of 1 mole electrolyte. Unit: S cm

^2

mol

^{-1}

Trend with Dilution —

Lambda_m

increases for both strong & weak electrolytes.

Strong Electrolytes —

Lambda_m

increases moderately due to reduced inter-ionic attractions. Follows Debye-Hückel-Onsager:

\Lambda_m = \Lambda_m^\circ - A\sqrt{c}

. Plot

Lambda_m

vs.

sqrt{c}

is linear.

Weak Electrolytes —

Lambda_m

increases sharply due to increased degree of dissociation (

alpha

). Plot

Lambda_m

vs.

sqrt{c}

is non-linear.

Degree of Dissociation ($alpha$) — For weak electrolytes,

\alpha = \frac{\Lambda_m}{\Lambda_m^\circ}

\alpha

increases with dilution.

Specific Conductivity Decreases, Molar Conductivity Increases (with dilution). Strong Electrolytes are Linear, Weak Electrolytes Sharp (on $\Lambda_m$ vs. $\sqrt{c}$ plot). Weak Dissociate More (on dilution).

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