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Molar Conductivity

Last Updated : 23 Jan, 2024
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Molar Conductivity is a basic physical chemistry concept that greatly impacts the understanding of electrical conduction in electrolyte solutions. This parameter denoted by the symbol Λm sheds light on this ion’s ability to transport current in solution. With the concentration of ions and their mobility, molar conductivity is a criterion for clarifying electrolyte behavior. Molar conductivity is important topic in class 12 Electrochemistry.

In this article, we will learn about Molar Conductivity, Formula for Molar Conductivity, Molar Conductance, and others in detail.

What Is Molar Conductivity?

Molar conductivity, (λ), is the magnitude of a solution that contains one mole of dissolved matter per liter volume. It is measured in Siemens per m or cm (S/m). Molar conductivity is a measure of the migrating ability of the molecules in solution due to an applied electric field.

Experimentally, the molar conductivity is evaluated by doing a measurement of conductance in a solution through the use of an instrument known as the conduction meter and subsequently dividing it by concentration which has been measured for that particular solution mol/L. Mathematically, it is given by the equation:

λ = κ / c

  • λ is Molar Conductivity
  • κ represents a solution in S/cm
  • c is Concentration of Solute

Variation of Molar Conductivity with Concentration

Molar Conductivity of various electrolytes is shown in the image added below,

Molar-Conductivity

Molar conductivity of both weak and strong electrolytes increases with a decrease in concentration or dilution.

Molar Conductivity Formula

The given expression below is used to represent Molar Conductivity mathematically.

Λm = K / C

where,

  • K is Specific Conductivity
  • C is Molar Concentration

Unit of Molar Conductivity

Molar Conductivity is measured in S⋅m²⋅mol⁻¹.

Molar Conductance

First we may define molar conductance as a specical property of every ion produced by any mole amount on an electrolyte in some volume solution.

It is given by the formula,

(Molar Conductance) μ = k.V

μ = k × 1000/c

Units of Molar Conductance: ω.cm².mol²

Equivalent Conductance = (Molar Conductance)/n

where,

n = (Molecular Mass/Equivalent Mass)

Molar conductance unit is Sm2mol-1.

Learn more about, Conductance of Electrolytic Solutions

Limiting Molar Conductivity

Molar conductivity at infinite dilution is the limiting one for a solution. Alternatively, when the concentration of electrolytes approximates zero, molar conductivity is referred to as a limiting value.

While studying the limiting molar conductivity of various strong electrolytes, Kohlrausch found specific tendencies. Based on his findings, Kohlrausch stated that the “limiting molar conductivity of an electrolyte can be expressed as a sum of individual contributions to charge carried by cation and anion.” This is referred to as the law for independent ion migration.

For instance, the limiting molar conductivity for sodium chloride can be computed from knowledge of the ion’s limited specific conductivities.

Importance of Molar Conductivity

Molar conductivity is an important parameter to understand the behaviour of electrolytes in solution. Electrolytes are compounds that break down into ions when dissolved in a solvent and the ability of these substances to conduct electricity is directly dependent on ion concentration and mobility. Molar conductivity plays an important role in chemical and electrochemical processes because it offers useful information on the dissociation stage for such a compound as well as ionic mobility.

Some of the key reasons why molar conductivity is important are:

  • Understanding Electrolyte Strength: Molar conductivity is utilized in gauging the intensity of an electrolyte, which represents the degree by which dissociation takes place to produce two charged particles. Ionic compounds that ionize almost completely include salts, acids and bases which have high molar conductivities.
  • Predicting Conductivity of Solutions: It is possible to predict the conductivity of different concentrations by using molar conductivity. The limiting molar conductivity (λ0) can be determined by the measurements of a solution’s molar conductivity at given concentration and applying extrapolation to zero concentration (infinite dilution).
  • Studying Ionic Mobility: Molar conductivity is proportional to the mobilities of ions in solution. It discusses the permeability of ions in a solution under an applied voltage.
  • Determining Equivalent Conductivity: The molar conductance of an electrolyte solution measures its equivalence conductivity, which is the measured value at a specified concentration.

Applications of Molar Conductivity

Molar conductivity is widely used in the chemical and electrochemistry Some of the notable applications are:

  • Determination of Dissociation Behavior: Molar conductivity refers to the degree of dissociation among electrolytes in solution. Determination of the degree of dissociation for an electrolyte can be achieved by measuring its molar conductivity across a range of concentrations.
  • Electrochemical Studies: Molar conductivity has a wide range of application in many electrochemical studies like, is kinetics, analytical techniques and corrosion.
  • Industrial Applications: Applications of Molar conductivity are industrial and include design if electrolytics cells for the deposition, electroplating metal and even remnant refining.
  • Pharmaceutical Industry: It is also used in the industry of medicines to determine how drugs behave and interact inside a solution. It has a role during drug design and formulation where it can help determining the solubility, stability and absorption of drugs in various formulations or scenarios.
  • Environmental Monitoring: Environmental monitoring uses molar conductivity to measure the electrical conductance of water samples taken from rivers, lakes and other bodies. It is one of the key criteria for evaluating the quality of water and determining concentrations in which salts, pollutants ,and other contaminants are dissolved into waters.

Specific Conductivity

The ability of an electrolytic solution to conduct the electric current is called the conductance of the solution. Conductance of the electrolytic solution is reciprocal of the resistance of that electrolytic solution. Specific conductivity of any electrolytic solution is measured in S (siemens).

Specific conductivity of an electrolytic solution depends on the various fctors that includes,

  • Nature and Concentration of Electrolyte in Solution
  • Size of the ions within Solution
  • Nature and its viscosity of Solvent
  • Temperature of Electrolytic Solution

Electrolytic Conduction

In an electrolyte the electricity is conduct not because of the free electrons as in case of metallic conductor but the current flows because of ions added in the solution. Arrhenius Theory is used to explain the movement of ions in an electrolyte. For an electrolyte AB its ion in water are formed as,

AB ↔ A+ + B_

Read More,

Problems 1: At 298 K, conductivity of 0.02 M solution of KCl is found out to be equal to 0.146 S cm^-1 Calculate the molar conductivity.

Solution :

Molarity (M) = 0.20M

Specific Conductance at 298 K is (k) = 0.0248 S.cm

Molar Conductivity = (100 × K) /M

= (100 × 2.48 ) / 2

= 124‬ cm² mol⁻¹

Thus, KCl solution has a molar conductivity of 124‬cm²mol⁻¹.

Problem 2: At infinity dilution, critical values of conductance for Na+ and Cl are: 51.12 × 10-4 Sm2mol-1 and 73.54 × 10-4 Sm2mol-1. Determine the overall molar conductance of NaCl.

Solution:

Molar conductance at infinite dilution

λ(Na+) = 51.121 × 10-4 Sm2mol-1

λ(Cl) = 73.54 × 10-4 Sm2mol-1

In terms of molar conductance, NaCl = λ(Na+) + λ(Cl)

= 51.12 × 10-4 + 73.54 × 10-4

= 124.66 × 10-4 Sm2mol-1

Hence, the molar conductivity of NaCl is 124.66 × 10-4 Sm2 mol².

Problem 3: The values of Molar conductances at infinite dilutions for Na + and Cl – ions are 32.54×10-2 Sm2.mol-1 and 67.12 × 10-2Sm2.mol-1. What is the complete Molar conductance of NaCl?

Solution :

Molar conductance at infinite dilution is calculated as follows:

λ(Na+) = 32.54 ×1 0-2Sm2.mol-1

λ(Cl) = 67.12 mM2 mol-1

Molar Conductance of NaCl = λ(Na+) + λ(Cl) = 32.54 × 10-2 +67.12 × 10-2

= 99.66 ×10-2 Sm2mol-1

As such, NaCl molar conductance equals 99.66 × 10-2Sm2mol-1.

Problem 4: Calculate the molar conductivity (Λom)​ for CaCl2​, given:

Λ Ca2​+ = 119.0 S.cm2mol-1

Λ Cl​ = 76.3 S.cm2mol-1

Solution:

Λom(CaCl2) = ΛCa2+ + 2×ΛCl

(Since CaCl2 consists of one calcium and two chloride ions, so we add their molar conductivity to get the same of the compound)

Λom(CaCl2) = 119.0 + 2×(76.3)

(Substituting the values of molar conductivity of the respective ions)

Λom(CaCl2) = 271.6 S.cm2mol-1.

Frequently Asked Questions (FAQs) Molar Conductivity

What is Molar Conductance of Weak Electrolytes look like when Diluted?

With dilution, the molar conductance of a weak electrolyte increases in response to larger dissociations.

Why do Electrolytes conduct Electricity?

Electrolytes conduct electricity because they have free ions.

What is Kohlrausch Law?

Kohlrausch’s law implies that in a dilute solution, the limiting molar conductivity of an electrolyte equals to summation of cations and anions limiting molar concentrates at infinite dissolution.

What is Connection between Molar Conductivity and Degree of Dissociation?

Molar conductivity is proportional to dissociation degree which means the fraction of total amount electrolyte that was degraded into ions.

Why is Limiting Molar Conductivity Important?

The measured limiting molar conductivity gives an estimate of how strong the electrolyte is and whether it dissociates readily as ions in a solution.

What Impact does Temperature have on Molar Conductivity?

The molar conductivity of a solution normally increases as temperatures rise due to greater ionic mobility, but there are some exceptions.

What is Limiting Molar Conductivity?

The molar conductivity limiting is the value at an infinite dilution of a solution.

What are k and c in Molar Conductivity?

k in Molar conductivity means Specific Conductance and c is the mole per liter concentration in Molar Conductance.

What is Specific Conductivity?

Specific conductance is measured electric current by the sample of an electrolyte solution.

What is SI Unit of Molar Conductivity?

SI Unit of Molar Conductivity is Sm2mol-1



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