Difference Between Specific Conductance and Molar Conductance
Electrolytic solutions let electricity flow by transporting charged ions. The primary keyword Electrolytic Conductance, Molar Conductance, and Specific Conductance is highly relevant for JEE because all three concepts—conductance, molar conductance, specific conductance—populate both conceptual and calculation-based questions. Knowing their formulas, behaviour with dilution, and differences helps you confidently tackle numericals in Electrochemistry and related topics. Pure water conducts electricity extremely poorly, but add a pinch of salt or acid, and ions carry charge efficiently—this real-life change underpins the ideas you’ll learn here.
Key Definitions: Conductance Types, Symbols & Units
The three commonly asked terms in this topic each describe how solutions conduct current but in different reference frames. Their units and symbols are critical for accurate problem-solving and avoiding confusion between similar-sounding quantities.
| Term / Symbol | Definition | SI Unit |
|---|---|---|
| Specific Conductance (κ) | Conductance of one cm3 of solution between electrodes 1 cm apart | S·cm–1 (Siemens/cm) |
| Molar Conductance (Λm) | Conductance of all ions from 1 mole electrolyte dissolved in solution | S·cm2·mol–1 |
| Equivalent Conductance (Λeq) | Conductance of all ions produced by 1 gram-equivalent of electrolyte | S·cm2·eq–1 |
Questions may also reference the term “electrolytic conductivity”, which is synonymous with specific conductance (κ).
Difference Between Specific Conductance and Molar Conductance
Specific conductance and molar conductance are deeply linked but not the same. JEE numericals test whether you can choose the right form by understanding their reference basis, formula, and practical use. Study this key comparison:
| Specific Conductance (κ) | Molar Conductance (Λm) |
|---|---|
| Refers to conductance of 1 cm3 solution | Refers to total conductance from 1 mole electrolyte |
| Depends on solution volume between electrodes | Depends on total ions furnished by mole in entire solution |
| SI: S·cm–1 Symbol: κ |
SI: S·cm2·mol–1 Symbol: Λm |
| Decreases with dilution (fewer ions/unit volume) | Increases with dilution (ions can move more freely) |
| Formula: κ = G × (l / A) | Formula: Λm = κ × (1000 / c) |
This table is ideal for quick revision and prevents common exam mistakes in distinction between specific conductance and molar conductance.
Key Mathematical Formulas and Units
JEE Main numericals almost always require use of correct variables, correct conversion of units, and precise application. Here are the critical equations:
- Specific Conductance (κ): κ = G × (l / A)
- Where G = measured conductance (S), l = distance between electrodes (cm), A = area (cm2)
- Molar Conductance (Λm): Λm = κ × (1000 / c)
- Where c = molar concentration (mol·L–1)
- Equivalent Conductance (Λeq): Λeq = κ × (1000 / C)
- Where C = normality (eq·L–1)
Remember conductance (G) has the unit Siemens (S), not ohm–1. Check your answers for dimensional consistency! Unit conversion pitfalls are common in exams, so always match the units to the required form.
Factors Affecting Conductance in Electrolyte Solutions
Three main factors influence all types of electrolytic conductance. This is crucial JEE theory and numerics:
- Concentration of solution: Specific conductance increases with concentration, but molar conductance increases with dilution due to free ion movement.
- Nature of electrolyte: Strong electrolytes fully dissociate, showing sharp conductance; weak electrolytes partially dissociate, so lower molar conductance at same c.
- Temperature: Higher temperature boosts ion mobility and thus increases all conductance values.
- Presence of impurities and solvent type can affect overall values.
In strong vs weak electrolytes, the variation of molar conductivity with dilution is especially important for both conceptual and graphical questions. You might be asked to compare the slope for strong (almost flat) versus weak (steeply rising) electrolytes for Λm versus √c graphs.
JEE Approach: Numericals & Application of Conductance Concepts
Understanding when to use κ, Λm, or Λeq is vital during JEE exams. Here’s a guide using solved forms straight from most asked question patterns:
- Identify the variable given and required (e.g., κ to Λm)
- Substitute directly using: Λm = κ × (1000 / c). Use c in mol·L–1, κ in S·cm–1.
- For equivalent conductance, replace c with C (eq·L–1).
- To calculate κ from measured cell resistance (R): κ = (cell constant) × (1 / R)
- Cell constant = l/A (geometric, in cm–1); plug into κ equation above.
Example: A solution with κ = 0.001 S·cm–1 at c = 0.01 mol·L–1.
Λm = 0.001 × (1000 / 0.01) = 100 S·cm2·mol–1.
Always confirm the SI units for final answers. Missing the (1000/c) factor in the molar conductance formula is a classic pitfall—write this constant down in your summary notes.
Quick Revision Table: Summary of Conductance Concepts
| Property | Specific Conductance | Molar Conductance | Equivalent Conductance |
|---|---|---|---|
| Symbol | κ | Λm | Λeq |
| SI Unit | S·cm–1 | S·cm2·mol–1 | S·cm2·eq–1 |
| Depends On | Volume/geometry of solution | Number of moles of electrolyte | Number of equivalents |
| Variation with Dilution | Decreases | Increases | Increases |
| Exam Formula | κ = G × (l/A) | Λm = κ × (1000/c) | Λeq = κ × (1000/C) |
Cross-link your learnings by exploring applications like electrochemical cells, differences between cell types, and foundational principles in redox reactions. Relating strong vs weak electrolyte conductance with molar conductivity questions is a high-frequency exam skill.
Vedantu’s study modules emphasize the difference between specific and molar conductance, with worked numericals and shortcuts. Mastering these concepts is a guaranteed booster for JEE Main Chemistry.
FAQs on Electrolytic Conductance, Molar Conductance, and Specific Conductance
1. What is electrolytic conductance?
Electrolytic conductance refers to the ability of an electrolyte solution to conduct electricity due to the movement of ions.
Key points:
- It measures how easily electric current passes through an electrolyte by ion migration.
- Depends on ion concentration, temperature, type of electrolyte, and solvent.
- Commonly measured in siemens (S) or mho (old unit).
2. What is the difference between specific conductance and molar conductance?
Specific conductance (κ) is the conductance of a solution contained between two electrodes 1 cm apart and 1 cm2 area, while molar conductance (Λm) is the conductance of all ions produced by one mole of electrolyte in a given volume of solution.
Key differences:
- Specific conductance refers to conductance per unit volume; molar conductance to conductance per mole.
- Specific conductance unit: S cm-1; Molar conductance unit: S cm2 mol-1.
- Specific conductance decreases with dilution; molar conductance increases as dilution increases.
3. What is the relationship between specific conductance and electrolytic conductance?
Specific conductance (κ) measures the total conductance of a unit volume of electrolyte, and it is directly related to electrolytic conductance.
Relationship:
- Electrolytic conductance refers generally to conductance by ionic solutions; specific conductance quantifies this conductance per cm3.
- Specific conductance (κ) = observed conductance (G) × cell constant (l/a)
4. What is the formula for molar conductance and its unit?
The formula for molar conductance (Λm) is:
Λm = κ × (1000 / C)
Where:
- κ = specific conductance (S cm-1)
- C = concentration in mol L-1
5. How does concentration affect conductivity?
Concentration affects conductivity because the number of ions per unit volume changes as you dilute or concentrate the solution.
Key effects:
- Specific conductance usually decreases with dilution, since there are fewer ions per unit volume.
- Molar conductance increases with dilution, as ion mobility increases and ion-ion interactions decrease.
- At infinite dilution, molar conductance reaches its maximum value (Λm0).
6. What is equivalent conductance and how is it different from molar conductance?
Equivalent conductance (Λeq) is the conductance of all ions produced by one gram-equivalent of an electrolyte in a certain volume.
Difference:
- Molar conductance relates to 1 mole of electrolyte; equivalent conductance to 1 equivalent.
- Formula: Λeq = κ × (1000 / N) where N is normality.
- Unit: S cm2 eq-1.
7. What is the unit of electrolytic conductivity?
The SI unit of electrolytic conductivity (specific conductance, κ) is siemens per meter (S m-1).
Other common units: S cm-1 (used in labs) or mho cm-1 (older unit).
8. What is the relationship between electrolytic conductivity and molar conductivity?
Molar conductivity (Λm) is derived from electrolytic conductivity (κ) by taking into account the concentration of the electrolyte.
Relationship:
- Λm = κ × (1000 / C), where C is concentration in mol L-1.
- Molar conductivity reflects conductance per mole of electrolyte for a given concentration.
9. How does molar conductivity differ between strong and weak electrolytes?
In strong electrolytes, molar conductivity increases slowly with dilution as they are fully ionized. In weak electrolytes, molar conductivity increases sharply with dilution due to increased ionization.
Important points:
- Strong electrolytes are almost completely dissociated at all concentrations.
- Weak electrolytes ionize more at higher dilution, causing a steep rise in Λm.
- Graphically, Λm vs √C is a straight line for strong and curves upward for weak electrolytes (Kohlrausch's Law).
10. How is molar conductivity calculated at infinite dilution?
Molar conductivity at infinite dilution (Λm0) is calculated using Kohlrausch’s Law of Independent Migration of Ions.
Calculation:
- Λm0 = λ0cation + λ0anion
- Add the limiting molar conductivities of individual ions at infinite dilution.
- This value helps find the dissociation constants and degree of ionization for weak electrolytes.
