Equivalent and Molar Conductance

 

Equivalent and Molar Conductance

            At any fixed temperature, the conductivity of a solution depends partly on the number of charges per unit volume which for different solutions may contain different amounts of electrolytic charges. Since the point of interest is to compare the current ability of a given number of electrolytic charges at different concentrations, it is not much rewarding to consider the conductivity/specific conductance as a fundamental quantity. In order to compare the conductance of an electrolyte with another electrolyte, the fundamental weight of an electrolyte must be dissolved in the same volume of the solution. This weight is either the equivalent weight or the molecular weight.

            The equivalent conductance of an electrolyte is defined as the conductance of a volume of the solution containing one gram equivalent of dissolved substance when placed between two parallel electrodes 1m apart, and large enough to contain between them all of the solutions. It is represented by ʌ (lambda) and never determined directly, but is calculated from the specific conductance. It represents the conductivity power of all the ions produced by dissolving one gram equivalent of the electrolytic at a given dilution and temperature.

ʌ   =Ls×V

            If C represents the concentration of a solution in gram equivalent per liter. Then the volume containing one equivalent of the electrolyte is 10-3/C m3. If Ls represents the specific conductance Sm-1, then the equivalent conductance is given as.

ʌ   =Ls.10^-3 /C

            The units of equivalent conductance are S m² eq^-1, but in common use, the last term is generally dropped out. The SI unit is S m².

            Another quantity that is quite often used is the molar conductance (µ)..it is defined as the conductance due to ions produced by one mole of the electrolyte at a given dilution and temperature.It may be defined as the conductance of a volume of a solution containing one mole of the electrolyte when placed between two parallel electrodes 1m apart and large enough to contain between them the whole of the solution. It is equal to the product of specific conductance (Ls) and volume of the solution containing one mole of the electrolyte.

µ =Ls× V

If C represents the concentration in mole per liter, then

µ =Ls.10^-3 /C S m² mol^-1 

ʌ Is related to the µ through the relation

ʌ= µ / Total no. of equivalents of cationic (or anionic) charges per mole of salt

or ʌ = µ/Z

Where Z = total positive charge carried by cations or total negative charge carried by anions furnished by a molecule of an electrolyte on dissociation. For example,

NaCL        Na⁺ +Cl^-                                          (Z=1)

CaCl₂          Ca²⁺ +2CL^-1                                   (Z=2)  

Al₂(SO₄)₃         2Al^{3+ +}3SO^2-_{4                      (}Z=6)

By: Muzalim Khalid

The writer is a post-graduate student from Chemistry Department University of Turbat

Turbat Kech Balochistan