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Question

Which of the following statement(s) is/are correct regarding the plot between Λm vs C?

Λm - Molar conductivity
C - concentration of electrolyte

A
Λm increase slowly with increase in dilution for strong electrolyte
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B
Λm increase steeply as dilution is increased, for weak electrolyte
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C
The plot curve is a straight line with negative slope for weak electrolyte
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D
Both (a) and (b)
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Solution

The correct option is D Both (a) and (b)
Variation in Λm for weak electrolyte:
Weak electrolytes like acetic acid have lower degree of dissociation.
With dilution of an ionic solution, the degree of dissociation (α) of the electrolyte increases which result in increase in the number of ions in solution. Near lower concentration, there will be a steep increase in molar conductance.

Plot of Λm vs C

From above it is evident that curve for Λm vs C for a weak electrolyte (here acetic acid) is rectangular hyperbola. It does not intercept the y-axis.

Variation in Λm for strong electrolyte:
For strong electrolyte Λm increases due to decreased ion- ion force of attraction.

As strong electrolytes are mostly in ionized state, no abrupt change in the number of ions with dilution.

Thus, Λm increases slowly with dilution.

In concentrated solution there will be a strong attraction between ions which slows down the movement of ion.

On dilution of strong electrolyte, distance between the ions increases which results in decrease in ion-ion attraction. So, there will be overall increase in molar conductance of the solution.

Variation in molar conductivity of strong electrolyte due to ion-ion interaction is given by an empirical relation proposed by kohlrausch.

Λm=Λ0mAC
Λm - Molar conductivity
Λ0m - Limiting molar conductivity
C is concentration of electrolyte
A is constant for a given solvent and temperature.

Λm=Λ0mAC
This equation is in the form of straight line equation,
y=c+mx

Plot of Λm vs C

Intercept : Λ0m
Slope : A
Thus, the extrapolation of line gives Λ0m for strong electrolytes.

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