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Transcript
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(f) Application of Kohlrausch law :
1. Molar conductivity of infinite dilution (L0) for weak electrolyte : The law can be
used to calculate the molar conductivity of any electrolyte at zero concentration. [However,
this law is particularly useful to calculate L0 of weak electrolytes for which the extrapolation
method (by graph) is not useful.]
Example :
L0 value of weak electrolyte, CH3COOH can be calculated from the L0 values of strong
electrolytes HCl, CH3COONa and NaCl.
L0 (HCl) + L0 (CH3COONa) â L0 (NaCl)
=
Î»0 + + Î»0
=
Î»0 + + Î»0
H
H
Cl â
+ Î»0
CH 3COO â
CH 3COO â
+ Î»0
âÎ» 0
Na +
Na +
â Î»0
Cl â
= L0 (CH3COOH)
Thus, L0 (CH3COOH) = L0 (HCl) + L0 (CH3COONa) â L0 (NaCl).
The L0 values of strong electrolytes can be calculated by extrapolation method and L0
of weak electrolytes can be evaluated.
(g) Relation between molar conductivity (L) and degree of dissociation
Î±) :
of weak electrolytes (Î±
1.
2.
L
L0
Where L is the molar conductivity of the weak electrolyte at the given concentration
C, and L0 is its molar conductivity at zero concentration.
The dissociation constant of a weak electrolyte (K) is given as,
Î±=
2
K
â´ K
Î± 2C
=
1â Î±
=
â L â
ââ L ââ C
0
=
L
1â
L0
2
â L â
ââ L ââ C
0
L0 â L
L0
=
L2
L20
Ã
L0
ÃC
( L 0 â L)
L2 C
=
L 0 ( L 0 â L)
(h) Measurement of conductivity :
The determination of conductivity (k) and molar conductivity (L) of a solution consists
of measurement of resistance of the solution by Wheatstone Bridge principle using
conductivity cell.
Chapter - 4 Electrochemistry
```
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