Conductivity or Specific Conductivity of an electrolyte solution is the measurement of its ability to conduct the electricity. Molar conductivity is the conducting power of the ions produced by dissolving one mole of an electrolyte in solution. It is denoted by Λ (lambda).
What is Conductance?
Conductance definition: Conductance the measurement of the easy flow of electricity along a certain path through an electrical element, like electric cable, bulb, etc. Electrical resistance is measured in terms of “Ohm”. It is denoted by R.
Where, R= resistance offered by the conductor, ρ, l, A are specific resistance, length and area of cross-section of the conducting material, respectively.
Unit of conductance: Conductance is considered to be the opposite of electrical resistance, i.e. mho or ohm-1. The SI unit is Siemens per meter (S/m) or ohm-1.
It is measured as ohm-1 = mho = Siemens per meter.
By substituting, R from equation (R= l/A. ρ), we get ( )
G = 1 /R
or G = 1/ (l/A. ρ) = κ. A/l equation (1)
where
κ = 1/ ρ= specific conductance, and ρ is the specific resistance of the conducting material,
From the equation (1),
If area of cross section is A = 1 cm2 and the length is l = 1 cm, then Kappa is equal to specific conductance, κ = G.
Therefore, the specific conductance can be defined as given here:
Specific conductance or conductivity, κ: It is the conductance of a conducting material or a solution between two electrodes of cross-sectional area 1 cm2 and separated by 1 cm distance.
Note that the above solution is occupying 1 cm3 volume.
It is measured in S.I. system as Siemens.m-1 (S/m).
The specific conductance depends on the nature of conducting substance or the electrolyte.
As the concentration of electrolytic solution increases the number of ions per unit area also increases, so does the specific conductance.
Cell constant (G*): The ratio of the distance between the electrodes “l” to the cross sectional area “A” of the electrodes is known as cell constant.
The cell constant can be determined by using following relations given below:
Based on the material’s electrical conductivity, the materials can be divided into two major types as mentioned below:
1. Insulators: The substances resisting the flow of electricity through them are called insulators. They lack free movement of the charged particles or free electrons. For example, glass, wood, organic polymers (like plastics), diamond, quartz, etc.
Conductors: The substances allowing the flow of electricity through them with slight resistance are called conductors.
They are again divided into:
I. Metallic and semi-metallic conductors: These are conductors which conduct the electricity through the electrons. For example, all the metals, graphite, etc.
In case of metallic conductors:
No chemical reaction occurs during the conduction of electricity.
With rise in temperature, due to vibrational disturbances among the molecules of a substance conductivity decreases.
II. Electrolytes: They are the substances or medium that give oppositely charged ions for the conduction of electricity. For example, NaCl, KCl, CH3COOH, HCl etc.
In case of electrolytes:
There is flow of ions towards the oppositely charged electrodes.
During conduction of electricity through electrolytes, oxidation occurs at anode, whereas reduction occurs at cathode, so the chemical reaction occurs.
The conductivity increases with increase in temperature as the extent of ionization also increases.
The electrolytes undergo dissociation to give ions either in molten state or in aqueous solutions. Depending on the extent of ionization (or dissociation) in water, the electrolytes are further divided into:
A. Strong electrolytes undergo complete ionization in water. A few examples are: NaCl, KCl, HCl, H2SO4, NaOH, etc.
B. Weak electrolytes undergo partial ionization in water. For example, HF, CH3COOH, NH4OH, HCOOH, etc.
III. Non-electrolytes: They are the substances that are unable to give ions or electrons for electrical conduction. For example, urea, glucose, sucrose, etc.
Molar Conductivity
It is defined as the conducting power of all the ions produced by 1 mole of an electrolyte in a given solution.
Molar conductivity (Λm or μ): The conductance of that volume of solution containing 1 mole of an electrolyte is known as molar conductivity. It is denoted by Λm or μ.
It is related to specific conductance, κ as:
μ = κ V
Or, μ = κ /M
where, M is the molar concentration.
If M is in the units of molarity i.e., moles per litre the Λ may be expressed as,
Λ = κ × 1000 / M
Consider a solution containing 1 gm mole of electrolyte placed between two parallel electrodes of 1 cm2 area of cross-section and one cm apart,
Conductance = Conductivity (μ) = Molar conductivity (Λ)
Now, if solution contains 1 gm mole of the electrolyte, then the measured conductance will be the molar conductivity. Thus,
Molar conductivity (μ) = 100 × Conductivity
In other words, (μ) = κ × V, where V is the volume of the solution in cm-1 containing one-gram mole of the electrolyte.
If M is the concentration of the solution in mole per litre, then M mole of electrolyte is present in 1000 cm -1
1 mole of electrolyte is present in 1000/M cm-1 of solution.
Thus, (μ) = κ × V in cm-1 containing 1 mole of electrolyte.
or μ = κ × 1000 / M
Units - ,
where M is molarity of the electrolytic solution.
Units of μ is: cm2.ohm-1.mol-1 = cm2.mho.mol-1, or m2.Siemens.mol-1.
The relation between equivalent conductance, Λ and molar conductance, μ can be given by:
μ = Λ × equivalent factor of the electrolyte
The equivalent factor of the electrolyte is usually the total charge on either anions or cations present in one formula unit of it. It may be equal to basicity in case of acids or equal to acidity in case of bases.
Factors Affecting the Conductance of Electrolyte Solution:
Temperature: The conductance of an electrolyte solution increases with increase in the temperature due to increase in the extent of ionization.
Nature of electrolyte: The strong electrolytes get completely ionized in water and therefore, show higher conductivities as they provide more numbers of ions. For example, HCl and HNO3.
Weak electrolytes undergo partial ionization in the presence of water and therefore show low conductivities in their solutions than strong electrolytes. For example, acetic acid (CH3COOH).
Ionic size and mobility: With increase in the size of ion, its mobility decreases, so the conductivity also decreases. For example, in molten state, the conductivity of sodium salts is greater than those of cesium salts as the size of Na+ ion is smaller than the Cs+ ion.
However, in aqueous solutions the extent of hydration affects the mobility of the ion, which in turn affect the conductivity. Heavily hydrated ions show low conductance values due to its larger size. For example, in aqueous solutions Na+ ion with high charge density is heavily hydrated than Cs+ ion with low charge density. Hence, hydrated Na+ is bigger than hydrated Cs+. As a result, sodium salts show lower conductivities compared to those of cesium salts in water.
The nature of solvent and its viscosity: In more viscous solvents, the ionic mobility gets reduced. So, the conductivity gets decreased.
Concentration: The specific conductance (κ) increases with increase in concentration of the solution (as the number of ions per unit volume increases).
Whereas, both the equivalent conductivity and molar conductance increase with decrease in concentration (i.e. upon dilution), as the extent of ionization also increases.
FAQs on Specific Conductivity and Molar Conductivity
1. What is meant by specific conductivity, and what does it measure?
Specific conductivity, also known as conductivity (κ), is a measure of a substance's ability to conduct electricity. It is defined as the conductance of a solution contained within a cell where the electrodes are of unit area and are separated by a unit distance. Essentially, it quantifies the conducting power of all the ions present in a unit volume (e.g., 1 cm³) of the solution.
2. What is molar conductivity, and how is it different from specific conductivity?
Molar conductivity (Λm) is the conducting power of all the ions produced by dissolving one mole of an electrolyte in a solution. It is a more useful measure for comparing the conductivity of different electrolytes.
The key differences are:
- Focus: Specific conductivity focuses on a fixed volume of solution, while molar conductivity focuses on a fixed amount (mole) of electrolyte.
- Dependence on Concentration: Specific conductivity generally decreases with dilution, whereas molar conductivity increases with dilution.
- Purpose: Specific conductivity measures the overall conductivity of a solution sample, while molar conductivity helps compare the efficiency of different electrolytes at conducting current.
3. What is the mathematical relationship between molar conductivity (Λm) and specific conductivity (κ)?
The relationship between molar conductivity and specific conductivity is given by the formula:
Λm = (κ × 1000) / C
Where:
- Λm is the molar conductivity (in S cm² mol⁻¹).
- κ (kappa) is the specific conductivity (in S cm⁻¹).
- C is the molar concentration of the electrolyte (in mol L⁻¹).
This formula is essential for converting between the two measures, as per the CBSE 2025-26 syllabus.
4. How does the dilution of an electrolyte solution affect its specific and molar conductivity?
Dilution (adding more solvent) has opposite effects on specific and molar conductivity:
- Specific Conductivity (κ) decreases: As you add more solvent, the concentration of ions per unit volume of the solution decreases. Since there are fewer charge carriers in any given cubic centimetre, the specific conductivity falls.
- Molar Conductivity (Λm) increases: Upon dilution, the total volume containing one mole of the electrolyte increases. This leads to greater separation between ions, reducing inter-ionic forces and increasing ionic mobility. For weak electrolytes, the degree of dissociation also increases. Both factors cause the molar conductivity to rise.
5. Why does specific conductivity decrease upon dilution while molar conductivity increases?
This is a fundamental concept. Think of it this way: Specific conductivity (κ) is like the traffic density on a specific 1 km stretch of a highway. As you widen the highway (dilution), the number of cars on that specific 1 km stretch decreases, so traffic density drops. In contrast, molar conductivity (Λm) is like the average speed of a fixed number of cars (1 mole of ions). When the highway widens (dilution), the cars can move much faster, so their average speed increases, even though the density at any single point is lower.
6. What are the standard units for specific conductivity and molar conductivity?
As per the SI system and NCERT guidelines, the units are:
- The SI unit for specific conductivity (κ) is Siemens per metre (S m⁻¹). However, the more commonly used unit in practice is Siemens per centimetre (S cm⁻¹).
- The SI unit for molar conductivity (Λm) is Siemens metre squared per mole (S m² mol⁻¹). The commonly used unit is Siemens centimetre squared per mole (S cm² mol⁻¹).
7. What is the significance of limiting molar conductivity (Λ°m)?
Limiting molar conductivity (Λ°m) represents the molar conductivity of an electrolyte solution at infinite dilution, meaning when its concentration approaches zero. At this point, inter-ionic attractions are negligible, and each ion contributes its maximum to the conductivity. Its significance lies in:
- Comparing Electrolytes: It provides a baseline to compare the conducting power of different electrolytes (strong vs. weak).
- Calculating Dissociation: For weak electrolytes, it is used in Kohlrausch's law to calculate their degree of dissociation (α = Λm / Λ°m).
8. How is the conductivity of an ionic solution measured experimentally?
The conductivity of an ionic solution is measured using a conductivity cell connected to a Wheatstone bridge. A pure direct current (DC) cannot be used as it would cause electrolysis and change the solution's composition. Instead, an alternating current (AC) is used to prevent the polarisation of electrodes. The resistance (R) of the solution is measured, and conductivity (κ) is calculated using the formula κ = (1/R) × (l/A), where (l/A) is the cell constant.
