Question

How are abnormal molecular masses related to the dissociation of a solute in solution?

How does van’t Hoff factor help in the determination of degree of dissociation of a solute in solution?

Answer 1

When a solute dissociates in solution (like electrolytes: NaCl, KNO₃, etc.), it produces more particles than expected. This leads to colligative properties (like osmotic pressure, boiling point elevation, and freezing point depression) being higher than normal, resulting in an abnormally low molecular mass. 

To account for this, we use the Van’t Hoff factor (i)

`i = "Observed colligative property"/"Calculated (normal) colligative property"`

`i = "Normal molecular mass"/"Observed molecular mass"`

For dissociation: i > 1

Then, i = 1 + α(n − 1)

`alpha = (i - 1)/(n - 1)`

More particles are formed, increasing colligative effects.

Observed molecular mass appears lower than actual.

Example: \[\ce{NaCl -> Na+ + Cl-}\]

Here, 1 mole gives 2 particles ⇒ i ≈ 2

Abnormal molecular masses occur due to dissociation of solute in solution. This increases the number of solute particles, enhancing colligative properties and leading to a lower observed molecular mass. The Van’t Hoff factor i corrects for this discrepancy.