Question

What do you understand by abnormal molecular mass?

Hint:

If molecular mass is higher than normal → association; If molecular mass is lower than normal → dissociation.

Answer 1

Step 1: Normal molecular mass.
Molecular mass of a solute is generally calculated from colligative properties (elevation of boiling point, depression of freezing point, osmotic pressure, relative lowering of vapor pressure). Ideally, this gives the correct molecular mass.
Step 2: Abnormal case.
Sometimes, the molecular mass determined experimentally using colligative properties differs from the theoretical (expected) value. This is called abnormal molecular mass.
Step 3: Causes of abnormal molecular mass.
- Association of molecules: Solute molecules combine to form aggregates (e.g., acetic acid in benzene). This leads to a molecular mass higher than expected.
- Dissociation of molecules: Solute molecules dissociate into ions (e.g., electrolytes like KCl in water). This leads to a molecular mass lower than expected.
Step 4: Explanation using van’t Hoff factor.
To correct this abnormal behavior, van’t Hoff introduced a factor \(i\): \[ i = \frac{\text{Observed colligative property}}{\text{Calculated colligative property}} \] Then, modified formula: \[ \Delta T_b = i \cdot K_b \cdot m \] \[ \Delta T_f = i \cdot K_f \cdot m \] Step 5: Conclusion.
Abnormal molecular mass occurs due to association or dissociation of solute molecules in solution. It is corrected using the van’t Hoff factor.