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ChemistryEdu Logo Electrochemistry | Variation of Conductivity and Molar Conductivity with Concentration#

Conductivity#

  • The conductivity of a solution at any given concentration is the conductance of one unit volume of solution kept between platinum electrodes with unit area of cross-section and at a distance of unit length.
G=ΚAl=Κ
  • Conductivity always decreases with decrease in concentration both for weak and strong electrolyte because the number of ions per unit volume that carry the current in a solution decreases on dilution.

Molar Conductivity#

  • Molar conductivity of a solution at a given concentration is the conductance of the volume V of solution containing one mole of electrolyte kept between two electrodes with area of cross section A and distance of unit length.

  • Molar conductivity is also defined as the conductivity of solution per unit its concentration.

λm=ΚC
λm=ΚnV
λm=ΚVn
  • Molar conductivity increases with decrease in concentration because λm1C.

  • It has been found that decrease in conductivity on dilution of a solution is more than compensated by increase in its volume. Physically, it means that at a given concentration, λm can be defined as the conductance of the electrolytic solution kept between the electrodes of a conductivity cell at a unit distance but having area of cross-section large enough to accommodate sufficient volume of solution that contains one mole of the electrolyte.

  • Limiting Molar Conductivity: When the concentration approaches zero, the molar conductivity is known as limiting molar conductivity and is represented by λm0.

  • The variation of molar conductivity, λm with concentration is different for strong and weak electrolytes.

Variation of Molar Conductivity with Concentration in Strong Electrolytes#

Variation of Molar Conductivity with Concentration in Strong Electrolytes

  • For strong electrolytes, λm increases slowly with dilution and can be represented by the equation:
λm=λm0AC1/2

Here, intercept = λm0 and slope = A

  • The value of 'A' for a given solvent and temperature depends on the type of electrolyte, ie, charges on the cation and anion produced on the dissociation of the electrolyte in the solution.

  • NaCl, CaCl2, MgSO4 are 1-1, 2-1 and 2-2 electrolytes respectively. All electrolytes of particular type have the same value for 'A'.

Kohlrausch Law of independent migration of ions#

  • It states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte.

  • In general, if an electrolyte on dissociation gives $ ν+ $ cations and $ ν- $ anions, then its limiting molar conductivity is given by:

λm0=ν+λ+0+νλ0
  • Here, λ+0 = Limiting molar conductivity of cation and λ0 = Limiting molar conductivity of anion

Variation of Molar Conductivity with Concentration in Weak Electrolytes#

Variation of Molar Conductivity with Concentration in Weak Electrolytes

  • For weak electrolytes, molar conductivity (λm) increases steeply on dilution, especially near lower concentrations. Therefore, limiting molar conductivity (λ0m) cannot be obtained by extrapolation of λm to zero concentration.

  • At infinite dilution (i.e, concentration → 0), electrolyte dissociates completely (α=1) but at such low concentration, the conductivity of the solution cannot be measured accurately.

  • Therefore, λ0m for weak electrolytes is obtained by using Kohlrausch law of independent migration of ions.

  • At any concentration C, if α is the degree of dissociation, then it can be approximated to the ratio of molar conductivity λm at the concentration C to limiting molar conductivity λ0m.

α=λmλm0
  • For weak electrolytes like acetic acid:
CH3COOHCH3COO+H+

At time t = 0, let concentration of CH3COOH = c and concentration of CH3COO- and H+ = 0.

If α is the degree of dissociation, at time t = t, concentration of CH3COOH = c-cα and concentration of CH3COO- and H+ = cα

Dissociation constant, Ka is given by:

Ka=[CH3COO][H+][CH3COOH]
Ka=cα×cαccα
Ka=cα2(1α)
where,α=λmλm0