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Molar and Equivalent Conductance is considered one the most difficult concept.
Molar Conductance at Infinite Dilution is considered one of the most asked concept.
45 Questions around this concept.
Limiting molar conductivity of is equal to:
An increase in equivalent conductance of a strong electrolyte with dilution is mainly due to:
The equivalent conductances of two strong electrolytes at infinite dilution in (where ions move freely through a solution ) at 25°C are given below:
What additional information/quantity one needs to calculate of an aqueous solution of acetic acid?
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Molar Conductance
The molar conductance is defined as the conductance of all the ions produced by the ionisation of 1 mole of an electrolyte when present in V ml of solution. It is denoted by Λm.
where V is the volume in ml containing 1 gm mole of the electrolyte.
If c is the concentration of the solution in mole per litre, then:
where c is the concentration of the solution in M. The units of are ohm-1 cm2 mol-1 or S cm2 mol-1 When the units of is S cm^{-1}.
It is to be noted that changing the units of the quantities involved will lead to a change in the formula. For the sake of homogeneity, when all the quantities are expressed in their SI unit.
Also, if AxBy is an electrolyte dissociating as:
Equivalent Conductance
One of the factors on which the conductance of an electrolytic solution depends is the concentration of the solution. In order to obtain comparable results for different electrolytes, it is necessary to take equivalent conductance.
It is defined as the conductance of all the ions produced by one gram equivalent of an electrolyte in a given solution. It is denoted by Λeq.
If ‘V’ is the volume in ml containing 1 gm equivalent of the electrolyte, the above equation can be written as:
Its units are ohm-1 cm2 equiv-1 or S cm2 equiv-1. A similar constraint of units exists in the formula as that in molar conductance.
Equivalent conductance is also given as follows:
Effect on conductance
Effect on degree of dissociation
Effect on Molar and Equivalent conductance
Both Λm and Λeq increases with dilution as conductance increases with dilution.
For strong electrolytes, the increase in Λm and Λeq is relatively small as increase in the number of molecules/ions is very small.
For weak electrolytes, the increase in Λm and Λeq is large and rapid as ? increases with dilution.
Effect on Conductivity
On dilution, number of molecules/ions per ml of the solution decreases. Since conductivity is defined as the conductance of one ml of the solution, conductivity decreases with dilution (due to a decrease in the conductance).
When addition of water doesn’t bring about any further change in the conductance of a solution, this situation is referred to as Infinte Dilution.
Strong Electrolytes: When infinite dilution is approached, the conductance of a solution of strong electrolyte approaches a limiting value and can be obtained by extrapolating the curve between Λm and c1/2 as shown in the figure given below:
The molar conductivity of strong electrolytes is found to vary with concentration as:
where b is a constant depending upon the type of electrolyte, the nature of the solvent and the temperature. This equation is known as Debye Huckel-Onsages equation and is found to hold good at low concentrations.
Weak Electrolytes: When infinite dilution is approached, the conductance of a solution of weak electrolyte increases very rapidly and thus, cannot be obtained through extrapolation. Also, the variation between Λm and c1/2 is not linear at low concentrations.
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