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Transcript
233
(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) – L0 (NaCl)
=
λ0 + + λ0
=
λ0 + + λ0
H
H
Cl –
+ λ0
CH 3COO –
CH 3COO –
+ λ0
–λ 0
Na +
Na +
– λ0
Cl –
= L0 (CH3COOH)
Thus, L0 (CH3COOH) = L0 (HCl) + L0 (CH3COONa) – 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
α) :
of weak electrolytes (α
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,
α=
2
K
∴ K
α 2C
=
1– α
=
⎛ L ⎞
⎜⎝ L ⎟⎠C
0
=
L
1–
L0
2
⎛ L ⎞
⎜⎝ L ⎟⎠C
0
L0 – L
L0
=
L2
L20
×
L0
×C
( L 0 – L)
L2 C
=
L 0 ( L 0 – 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