Equivalent Conductance
• It is the conductance of one gram equivalent of the electrolyte
dissolved in V cc of the solution.
• Equivalent conductance is represented by λeq
λeq = k x V
N= no. of gram eq / v(ml) X
1000
λeq = k x (1000/ N)
Where, k = Specific conductance, V = Volume of solution in cc. containing
one gram equivalent of the electrolyte and N is normality.
• In general if an electrolyte solution contains N gram equivalents in
1,000 cc of the solution the volume of the solution containing 1 gram
equivalent will be 1000/N x 106 m3 (1 cc= 10-6 m3 )
• λeq = k 10-3 / N mho.m3.(gm.equiv)-1
• for 1 : 1 electrolyte normality N equals to molarity 'C'. Then λ eq = k
10-3 / M mho.m3.(mol)-1
• λ eq values depend on the type of the electrolyte, concentration of
the solution and temperature.
Molar conductance
• It is the conductance of a solution containing 1 mole of the
electrolyte in V equal 1cm3 of solution. it is represented as m.
λm = K x V
λm =k x 1000/M
Where V = volume solution in cc
λm = Molar conductance
k = conductivity
M=molarity of the solution.
• For 1 : 1 electrolyte like NaCl, equivalent conductance is equal to molar
conductance.
Example
The resistance of 0.01N NaCl solution at 250C is 200
ohm. Cell constant of conductivity cell is unity. Calculate
the equivalent conductance and molar conductance of
the solution.
Solution:
Conductance of the cell=1/resistance =1/200 =0.005 S.
Specific conductance=conductance x cell constant =0.005 x 1
=0.005 S cm-1
Equivalent Conductance = Specific conductance x (1000/N)
= 0.005 x 1000/0.01
= 500 ohm-1cm2eq-1
Molar Conductivity = Equivalent conductivity x n-factor
= 500 x 1
= 500 ohm-1mol-1cm2
Q) The resistance of a 0.01 N solution of an electrolyte was found to
210 ohm at 25oC. using a conductance cell with a cell constant 0.88
cm-1. Calculate the specific conductance and equivalent conductance
of the solution.
Solution
Factors effect on The magnitude of the conductivity
exhibited by the ionic solution
1. Nature of the electrolyte.
2. number of solvated ions free to move in solution. Variation of
conductivity with concentration c of ionic solution can be used
to distinguish between true and potential electrolytes.
3. Temperature.
4. Mobility (related to speed) of ions in solution phase.
5. Charge on ion.
1) Nature of electrolyte :
• The conductance of an electrolyte depends upon the number of ions
present in the solution. Therefore, the greater the number of ions in
the solution the greater is the conductance.
• The strong electrolytes dissociate almost completely into ions in
solutions and, therefore, their solutions have high conductance.
• On the other hand, weak electrolytes, dissociate to only small extents
and give lesser number of ions so they have low conductance.
• The ionic mobility is reduced in more viscous solvents. Hence the
conductivity decreases.
2) Concentration of the solution :
• The molar conductance of electrolytic solution varies with the
concentration of the electrolyte.
• In general, the molar conductance of an electrolyte increases with
decrease in concentration or increase in dilution.
C HCl KCl KNO3 CH3COOH NH4OH
0.1 391.3 129.0 120.4 5.2 3.6
0.05 399.1 133.4 126.3 – –
0.01 412.0 141.3 132.8 16.3 11.3
0.005 415.8 143.5 131.5 – –
0.001 421.4 146.9 141.8 49.2 34.0
0.0005 422.7 147.8 142.8 67.7 46.9
(Infinite dilution) 426.2 149.9 146.0 390.7 271.0
• Inspection of table reveals that the molar conductance of strong
electrolyte (HCl, KCl, KNO3) as well as weak electrolytes (CH3COOH,
NH4OH) increase with decrease in concentration or increase in
dilution.
The variation is however different for strong and weak electrolytes.
(i) Variation of conductivity with concentration for strong
• It has been observed that the
variation of molar conductivity with
concentration may be given by the
expression
Λ = Λ0 – Ac1/2
where, A is a constant and Λ0 is called molar
conductivity at infinite dilution.
• For strong electrolytes, there is no increase in the number of ions with
dilution because strong electrolytes are completely ionised in solution at
all concentrations .
• However, in concentrated solutions of strong electrolytes there are strong
forces of attraction between the ions of opposite charges called inter-
ionic forces.
• Due to these inter-ionic forces the conducting ability of the ions is less in
concentrated solutions.
• With dilution, the ions become far apart from one another and
inter-ionic forces decrease. As a result, molar conductivity
increases with dilution.
• When the concentration of the solution becomes very-very low, the
inter-ionic attractions become negligible and the molar conductance
approaches the limiting value called molar conductance at infinite
dilution.
• This value is characteristic of each electrolyte.
(ii) Variation of molar conductivity with concentration for
weak electrolytes :
• The weak electrolytes dissociate to a much lesser extent as
compared to strong electrolytes .
• Therefore, the molar conductivity is low
as compared to that of strong
electrolytes.
• However, the variation of Λm with √c is
very large, we cannot obtain molar
conductance at infinite dilution (Λ0 (by
extrapolation of the Λm versus √c plots .
