When substances like salt or acid dissolve in water, they break into ions that can carry electricity. Molar conductivity hel;ps us understand how well these ions move and conduct electricity when just one mole of the substance is present in the solution. It’s a useful concept in chemistry, especially when studying how strong or weak electrolytes are. Whether you’re testing the purity of water or figuring out how batteries work, molar conductivity gives important clues about how efficiently electricity can flow through a solution.

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What is Molar Conductivity?

Molar Conductivity is the measure of a solution’s ability to conduct electricity when a mole of an electrolyte is dissolved in it. It tells us how effectively the ions produced from a mole can carry electric current. The unit of molar conductivity is Siemens metre squared per mole (S·m²·mol⁻¹), but it’s often expressed as S·m²·mol⁻¹ in practice. This concept helps chemists compare how different substances behave in solution and how their ion movement changes with dilution.

The mathematical representation of molar conductivity can be expressed using the following formula: Λm = K / C

Here, 'K' represents the specific conductivity, and 'C' stands for the concentration in moles per litre.

Generally, the molar conductivity of an electrolytic solution is defined as the conductance of the volume of the solution that contains one mole of the electrolyte. This is measured between two electrodes of a unit area cross-section or at a distance of one centimetre apart.

The unit of molar conductivity is S⋅m2⋅mol-1.
Learn more about the thermal conductivity of materials and how heat is transferred through them.

Variation of Molar Conductivity with Concentration

Molar conductivity changes as the concentration of a solution changes. When a solution is more concentrated, the ions are closer together and don't move as freely, so molar conductivity is lower. But as you dilute the solution by adding more water, the ions get farther apart and can move more easily, causing molar conductivity to increase. This means that molar conductivity usually goes up when the solution becomes weaker or more diluted. Understanding this helps us know how substances behave in different concentrations, especially when studying strong and weak electrolytes.

Molar Conductivity Numericals Problems 

Problem 1: Calculating the molar conductivity of a KCl solution.

Given:

Molarity (M) = 0.30M

Conductivity at 298 K (k) = 0.023 S cm−1

Solution:

The formula for molar conductivity is Λm = (1000 × k) / M.

Substituting the given values, we get Λm = (1000 × 0.023) / 0.30.

Simplifying, we get Λm = 76.66 cm² mol⁻¹.

Therefore, the molar conductivity of the KCl solution is 76.66 cm² mol⁻¹.

Problem 2: The conductivity of a 0.02 M solution of KCl at 298 K is 0.0248 S cm−1. Calculate the molar conductivity.

Given:

Molarity (M) = 0.20M

Conductivity at 298 K (k) = 0.0248 S cm−1

Solution:

Using the formula for molar conductivity, Λm = (1000 × k) / M.

Substituting the given values, we get Λm = (1000 × 0.0248) / 0.20.

Simplifying, we get Λm = 124 cm² mol⁻¹.

Therefore, the molar conductivity of the KCl solution is 124 cm² mol⁻¹.

Problem 3: The molar conductance values at infinite dilution of Na+ and Cl- ions are 51.12 × 10-4 Sm2 mol-1 and 73.54× 10-4 Sm2 mol-1, respectively. Calculate the total molar conductance of NaCl.

Solution:

We know that the molar conductance at infinite dilution for Na+ (λ+Na) is 51.12×10−4 Sm2 mol−1 and for Cl- (λ+Cl) is 73.54×10−4 Sm2 mol−1.

Therefore, the molar conductance of NaCl can be calculated as λ+Na + λ+Cl = 51.12×10−4 + 73.54×10−4.

Simplifying, we get 124.66×10−4 Sm2 mol−1.

Therefore, the molar conductance of NaCl is 124.66 ×10−4 Sm2 mol−1.

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