Molar Conductivity

In chemistry, molar conductivity is the conductance of a volume of solution containing 1 mole of dissolved electrolyte, when placed between two parallel electrodes. 

What is Molar Conductivity? 

Molar Conductivity can be termed as the conductance property or conducting power of all the ions that are drawn up by breaking down a mole of electrolyte in a solution. It has the property of an electrolyte solution that is used in specifying the efficiency of an electrolyte in conducting the electricity in a solution. Hence, it’s not a constant. 

Molar Conductivity Formula

The expression given below represents the mathematically molar conductivity formula.

Λm = K / C

Thus, where K = the particular conductivity;

C = the concentration in mole per litre.

Broadly, the molar conductivity of an electrolytic solution is the conductance of the volume of the solution comprising a unit mole of electrolyte that is positioned between two electrodes of unit area cross-section or at a range of one centimetre apart. 

Therefore, the unit of molar conductivity is S⋅m2⋅mol-1.

Variation of Molar Conductivity with concentration 

Weak and strong electrolytes of the molar conductivity increase with a decrease in concentration or dilution. As you learned previously, molar conductivity is the conductivity given by one mole of ions. Just after dilution, it will still consider the exact unit mole of ions. Yet, the increased dilution impacts dissociation of further electrolytes into ions and effectively increases the number of active ions in the solution. Therefore, this imparts more conductivity.

 The above graph illustrates that for strong electrolytes, the molar conductivity is increasing gradually with the dilution. If Eom is the restricting molar conductivity i.e., the molar conductivity at zero concentration, accordingly the general equation for the graph of the strong electrolyte can be conveyed via Kohlrausch’s law. 

Λm = Eo m – A √ c

Where,

A = the slope of the plot. 

It depends upon the kind of electrolyte at a given temperature for a given solvent.

Weak electrolytes, nonetheless, the molar conductivity stays at lower concentrations. Contrarily, such electrolytes possess lower molar conductivity at higher concentrations because there is a reduced degree of dissociation.

In specific conductivity, it is recognized that the conductivity increases as the concentration of the electrolyte increases. It depends upon the number of ions present in the unit volume of the solution. On dilution, the dissociation increases, affecting the current-carrying ions to increase in the solution. But just because of dilution, the number of ions present in a unit volume of the solution decrease. This results in a reduction in conductivity.

In the case of Strong electrolytes, the increase in concentration stimulates a sharp increase in conductivity. Nevertheless, at lower concentrations, weak electrolytes retain very low values of specific conductivity and the value deliberately increases as the concentration is increased. This happened as the increase in the number of active ions in the solution due to concentration.

Specific Conductivity 

Specific conductivity is the measure of the ability of a material to conduct electricity. It is represented by the symbol “K”. Hence, by the definition, G=R=p.

The extent to which a given sample of electrolytic solution can conduct an electric current is called its conductance, whereas the converse of the extent to which the given sample of electrolytic solution can resist the flow of electric current is called its resistance. So it can be concluded that the conductance is the reciprocal of resistance of that particular electrolytic solution. SI unit would be of conductance S (Siemens).

The SI unit of specific conductivity is Siemens per meter, better represented as S/m. 

Depending on factors of specific conductivity of an electrolytic solution 

• The concentration and nature of the electrolyte in the solution

• Size of ions within the solution

• Nature of solvent and its viscosity

• The temperature of the electrolytic solution 

Therefore different electrolytic solutions have different concentrations and contain different numbers of ions. Due to this factor, specific conductivity is not a suitable quantity to compare the conductance of different electrolytic solutions. Due to this, molar conductivity is introduced.

Electrolytic Conduction

When a voltage is applied to the electrodes dipped into an electrolytic solution, ions of the electrolyte move, and, therefore, electric current flows through the electrolytic solution.

The power of the electrolytes to conduct electric current is termed conductance or conductivity. Electrolytic solutions obey Ohm’s law.

It is based on the Arrhenius equation or theory that explains when a neutral electrolyte is dissolved in water, the electrolyte molecules split into differently charged ions. Positive ions are called cation and negative are called anions respectively. The ions are already present in the electrolyte, however, bound by electrostatic force and hence they are neutral. 

Equivalent conductivity in electrolytic conductance

As we know that electrolytic conduction is termed as the conducting power of all the ions produced by dissolving 1 g equivalent of an electrolyte in solution however the equivalent conductivity is defined as the conductance of a volume of solution containing one equivalent weight of dissolved substance when placed between two parallel electrodes 1cm apart and large enough to contain the entire solution between them.

Conclusion

Hence, Molar conductivity is the particular conductivity that is divided by the concentration in mole per litre where two electrodes are present with a positive and negative charge, respectively of unit area cross-section of 1-cm apart.