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CL 201 Date : Batch: ……..……...

Name : ………………………………….. Roll No.: …………..

EXPERIMENT 9

VERIFICATION OF DEBYE-HÜCKEL-ONSAGER EQUATION

FOR STRONG ELECTROLYTES

AIM

To verify the Debye-Hückel-Onsager equation λc = λo – (A + B λo)√c

for a strong uni-univalent electrolyte (KCl).

THEORY

Solutions of electrolytes (acids, bases and salts in water) conduct

electricity. In these solutions, the passage of electric current is due to

the migration of cations and anions in opposite directions.

It is the valence, migration velocity and the concentration of the ions

which determine the strength of the current flowing through a

solution of an electrolyte.

According to the Arrhenius theory, strong electrolytes (KCl, NaCl)

dissociate completely at all dilutions.

The increase in the molar conductance of solutions of strong

electrolytes, with dilution, is mainly due to the increased mobility of

the ions. Increase in dilution results in a decrease of inter ionic

interactions, thereby increasing the migration speed of the ions, and

hence the equivalent conductivity.

The conductance behavior of strong electrolytes has been given by

the Debye-Hückel-Onsager equation λc = λo – (A + B λo) √c ,

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where λc is the equivalent conductivity at concentration c ,

λo is the equivalent conductivity at infinite dilution,

A and B are the Debye – Hückel – Onsager coefficients.

At 25oC , the values are A = 60.2 , and B = 0.229 .

A plot of λc (y - axis) against √c (x - axis) should be linear,

with a slope = – (A + B λo).

The intercept on the y-axis would give λo .

Hence, λo can be obtained by extrapolating the graph to zero

concentration.

MATERIALS REQUIRED

Conductometer (with cell), pipettes, beakers, KCl, volumetric flasks.

PROCEDURE 1. Prepare a stock solution of 0.1 N KCl solution by exact

weighing.

2. Standardize the conductometer using this 0.1 N KCl solution.

3. Measure the conductance of the 0.1 N KCl solution.

4. Dilute the stock solution of 0.1 N KCl, so as to obtain

solutions of the following concentrations of KCl :

0.05 N ; 0.025 N ; 0.0125 N ; 0.00625 N ; 0.00312 N .

5. Measure the conductance of each solution prepared in Step 4.

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OBSERVATIONS Room Temperature = …….. oC Table 1. Conductivity of KCl solutions ____________________________________________________________

Sl. No. Concentration √c specific conductance

of KCl (N)

(c) ____________________________________________________________

1 0.10 2 0.05 3 0.025 4 0.0125 5 0.00625

6 0.00312 ____________________________________________________________

CALCULATIONS AND GRAPH

Plot a graph of λc (y - axis) against √c (x - axis).

Extrapolate to C = 0

Calculate the value of the Slope and the Intercept.

Comment on the values obtained.

RESULTS