Question

Which of the following statement is incorrect ?

${\Lambda }_m$ - Molar conductivity
${\Lambda }_m^0$ - Limiting molar conductivity
$C$ - concentration of electrolyte

• A. Molar conductivity of a strong electrolyte increases with dilution
• B. Variation of molar conductivity for strong electrolyte is given by,
  ${\Lambda }_m={\Lambda }_m^0+\sqrt{C}$
• C. Molar conductivity of a weak electrolyte increases with dilution
• D. ${\Lambda }_m^0$ for a strong electrolyte can be found from the plot of ${\Lambda }_{m}vs\sqrt{C}$by extrapolation of ${\Lambda }_m$ to zero concentration

Answer 1

The correct option is B Variation of molar conductivity for strong electrolyte is given by,

${\Lambda }_m={\Lambda }_m^0+\sqrt{C}$
On dilution, the molar conductivity increases for both strong and weak electrolyte.

For weak electrolyte it increase due to the increase in the degree of dissociation of the electrolyte.

For strong electrolyte it 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, ${\Lambda }_m$ 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.

${\Lambda }_m={\Lambda }_m^0-A\sqrt{C}$
${\Lambda }_m$ - Molar conductivity
${\Lambda }_m^0$ - Limiting molar conductivity
$C$ is concentration of electrolyte
$A$ is constant for a given solvent and temperature.

${\Lambda }_m={\Lambda }_m^0-A\sqrt{C}$
This equation is in the form of straight line equation,
$y=c+mx$
$\therefore$
Plot of ${\Lambda }_{m}vs\sqrt{C}$


Intercept : ${\Lambda }_m^0$
Slope : $-A$
Thus, the extrapolation of line gives ${\Lambda }_m^0$ for strong electrolytes.