Molar conductivity (Λm) is the conductance of the volume of solution containing one mole of an electrolyte, placed between two electrodes one unit distance apart and large enough to contain the solution.
where:
- Λm = molar conductivity
- κ = conductivity
- C = concentration in mol L⁻¹
The SI unit of molar conductivity is S m² mol⁻¹. It is also commonly expressed as S cm² mol⁻¹.
Strong Electrolytes
Strong electrolytes such as HCl, KCl, NaCl and HNO₃ are almost completely ionised in solution even at relatively higher concentrations.
When the solution is diluted, the ions experience less interionic attraction and their mobility increases.
For strong electrolytes, the increase in molar conductivity on dilution is relatively small and gradual.
At infinite dilution, molar conductivity reaches its maximum value, called Λ°m.
This is the approximate relation given by Kohlrausch for strong electrolytes.
Why does Λm increase?
- Dilution decreases interionic attraction.
- Ion mobility increases.
- Therefore, the molar conductivity increases.
Weak Electrolytes
Weak electrolytes such as CH₃COOH, NH₄OH and other weak acids and bases are only partially ionised in solution.
When the solution is diluted, the degree of ionisation increases significantly.
Therefore, molar conductivity increases very sharply on dilution.
The increase is much greater than that observed for strong electrolytes.
Why is the increase so large?
- Weak electrolytes are incompletely ionised.
- Dilution shifts the ionisation equilibrium towards ions.
- The number of ions increases greatly.
- Ion mobility also increases.
- Therefore, Λm rises sharply.
| Property | Strong Electrolyte | Weak Electrolyte |
|---|---|---|
| Ionisation | Almost complete | Partial |
| Examples | HCl, NaCl, KCl | CH₃COOH, NH₄OH |
| Effect of dilution | Small increase | Large increase |
| Ionisation on dilution | Changes little | Increases greatly |
| Graph | Gradual increase | Steep increase |
| At infinite dilution | Λ°m | Λ°m |
Molar conductivity increases on dilution for both strong and weak electrolytes, but the increase is small for strong electrolytes and very large for weak electrolytes.
Variation of Molar Conductivity with √Concentration
Strong electrolyte: nearly linear decrease of Λm with √C.
Weak electrolyte: highly curved graph because dilution causes a large increase in ionisation.
Q. How does molar conductivity change with dilution?
Answer:
Molar conductivity increases with dilution for both strong and weak electrolytes. The increase is small for strong electrolytes but large for weak electrolytes because dilution greatly increases their degree of ionisation.
Q. Explain the variation of molar conductivity of strong electrolytes with concentration.
Answer:
- Strong electrolytes are almost completely ionised.
- On dilution, interionic attraction decreases.
- Ion mobility increases, so molar conductivity increases.
- The increase is relatively small and gradual.
Q. Compare the variation of molar conductivity with concentration for strong and weak electrolytes.
| Strong Electrolytes | Weak Electrolytes |
|---|---|
| Almost completely ionised. | Partially ionised. |
| Dilution causes a small increase in Λm. | Dilution causes a large increase in Λm. |
| Ionisation changes little. | Ionisation increases greatly. |
| Graph is approximately linear with √C. | Graph is strongly curved. |
Q. Explain the variation of molar conductivity with concentration for strong and weak electrolytes with suitable examples.
Strong Electrolytes
Strong electrolytes such as HCl, NaCl and KCl are almost completely ionised. On dilution, the interionic attraction decreases and the mobility of ions increases. Hence molar conductivity increases gradually.
Weak Electrolytes
Weak electrolytes such as CH₃COOH and NH₄OH are only partially ionised. Dilution increases their degree of ionisation considerably. Consequently, the number of ions and their mobility increase, producing a sharp rise in molar conductivity.
Λm increases on dilution for both types, but the increase is much greater for weak electrolytes.
Q. Explain in detail how molar conductivity varies with concentration for strong and weak electrolytes. Give the reason for the difference.
1. Strong Electrolytes
Strong electrolytes such as HCl, HNO₃, NaCl and KCl are almost completely ionised even at appreciable concentrations.
When concentration is decreased, the ions become farther apart and interionic attraction decreases. Consequently, ionic mobility increases and molar conductivity increases.
The increase is relatively small because the electrolyte is already almost completely ionised.
2. Weak Electrolytes
Weak electrolytes such as CH₃COOH and NH₄OH are only partially ionised. When the solution is diluted, their degree of ionisation increases substantially.
Therefore, molar conductivity increases sharply on dilution.
3. Mathematical Relation
For strong electrolytes, Kohlrausch's limiting-law form is approximately:
where Λ°m is the molar conductivity at infinite dilution and K is a constant for a particular electrolyte at a given temperature.
4. Main Difference
| Strong | Weak |
|---|---|
| Already almost completely ionised | Partially ionised |
| Small increase on dilution | Large increase on dilution |
| Mainly due to increased ionic mobility | Due to both increased ionisation and mobility |
For both strong and weak electrolytes, molar conductivity increases as concentration decreases. However, the increase is gradual for strong electrolytes and very steep for weak electrolytes because dilution greatly increases the ionisation of weak electrolytes.
1. Molar conductivity is represented by:
✅ Answer
B) Λm
2. The relation between conductivity and molar conductivity is:
✅ Answer
B
3. On dilution, molar conductivity generally:
✅ Answer
B) Increases
4. Strong electrolytes are:
✅ Answer
B
5. Which is a strong electrolyte?
✅ Answer
C) HCl
6. Which is a weak electrolyte?
✅ Answer
D) CH₃COOH
7. For strong electrolytes, Λm increases on dilution:
✅ Answer
B
8. For weak electrolytes, Λm increases on dilution:
✅ Answer
A
9. The large increase in Λm for weak electrolytes is mainly due to:
✅ Answer
B
10. At infinite dilution, molar conductivity is called:
✅ Answer
B
11. Kohlrausch's relation for strong electrolytes involves:
✅ Answer
A) √C
12. On dilution, interionic attraction generally:
✅ Answer
B
13. Increased ionic mobility causes molar conductivity to:
✅ Answer
A
14. Which electrolyte shows the steepest increase in Λm on dilution?
✅ Answer
C) CH₃COOH
15. Weak electrolytes have:
✅ Answer
B
16. Which statement is correct?
✅ Answer
B
17. The main reason for gradual increase in strong electrolytes is:
✅ Answer
B
18. The unit S cm² mol⁻¹ is commonly used for:
✅ Answer
B
19. Which pair contains a strong and a weak electrolyte respectively?
✅ Answer
A
20. The maximum molar conductivity is obtained at:
✅ Answer
B) Infinite dilution
🎯 Quick Revision
Weak electrolyte: Partially ionised → dilution greatly increases ionisation → sharp increase in Λm
At infinite dilution: Λ°m is maximum
Strong electrolyte relation: Λm = Λ°m − K√C
