Abnormal Molar Mass

In an ideal scenario, the molar mass of a solute can be accurately determined using colligative properties such as elevation in boiling point, depression in freezing point, relative lowering of vapor pressure, and osmotic pressure. However, in certain cases, the experimentally determined molar mass differs significantly from the theoretically expected value. This deviation leads to what is known as Abnormal Molar Mass.

⚠️ Definition: When the molar mass of a solute determined through colligative properties is different from its actual (theoretical) molar mass, it is called an abnormal molar mass.

Causes of Abnormal Molar Mass

• Association: Molecules form aggregates (e.g., acetic acid in benzene), reducing particle number and increasing molar mass.
• Dissociation: Molecules split into ions (e.g., NaCl in water), increasing particle number and decreasing molar mass.

Van’t Hoff Factor (𝑖)

The Van’t Hoff factor (𝑖) helps correct the molar mass by accounting for the number of particles in solution.

Formulas:
𝑖 = Observed Colligative Property / Expected Colligative Property
or, 𝑖 = Normal Molar Mass / Abnormal Molar Mass

Examples

• Association: Acetic acid in benzene forms dimers → fewer particles → lower colligative effect → higher molar mass.
• Dissociation: NaCl dissociates into Na⁺ & Cl⁻ → more particles → higher colligative effect → lower molar mass.

Correcting Abnormal Molar Mass

• Determine the colligative property (ΔT_f, π, etc.)
• Calculate or estimate Van’t Hoff factor (𝑖)
• Use corrected formula to find true molar mass

Conclusion

Abnormal molar mass occurs due to the association or dissociation of solute particles in solution. The Van’t Hoff factor provides a reliable correction mechanism to determine the true molar mass using colligative properties.

Multiple Choice Questions (MCQs)

1. What causes abnormal molar mass in NaCl solution?
   A. Association
   B. Dissociation ✅
   C. Aggregation
   D. Sublimation
2. The Van’t Hoff factor for complete dissociation of NaCl is:
   A. 1
   B. 2 ✅
   C. 0.5
   D. 3
3. Which of the following will show higher observed molar mass?
   A. HCl in water
   B. NaCl in water
   C. Acetic acid in benzene ✅
   D. KCl in water

True or False (with Explanations)

1. Association leads to a higher colligative effect. False
   ➤ Explanation: It reduces the number of particles, so the colligative effect decreases.
2. Van’t Hoff factor helps determine the true molar mass. True
3. Abnormal molar mass only occurs in ionic compounds. False
   ➤ Explanation: It can occur due to association in covalent compounds too (e.g., acetic acid).

Frequently Asked Questions (FAQs)

Q1. What is abnormal molar mass?
A: It is the molar mass determined from colligative properties that deviates from the theoretical value due to association or dissociation.

Q2. How does Van’t Hoff factor correct abnormal molar mass?
A: It accounts for particle changes due to association/dissociation, helping adjust the molar mass calculation.

Q3. Which compounds show abnormal molar mass?
A: Ionic compounds like NaCl (dissociation) and covalent compounds like acetic acid (association) show this effect.