Question

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

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

Answer 1

When a solute associates in solution (e.g., carboxylic acids in benzene forming dimers), two or more molecules combine to form larger units. This reduces the number of particles in solution, leading to lower colligative properties and thus an abnormally high molecular mass.

To explain this, we use the Van’t Hoff factor (i):

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

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

For association, i < 1

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

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

Fewer particles are present due to molecule combination.

Colligative properties are reduced.

Observed molecular mass appears higher than actual.

Example:
Acetic acid in benzene associates to form dimers:

\[\ce{2CH3COOH -> (CH3COOH)2}\]

Only half the number of particles are present compared to the unassociated state.

Abnormal molecular masses occur due to the association of solute molecules in solution, decreasing particle count and colligative effects, thus showing a higher observed molecular mass. The Van’t Hoff factor i < 1 accounts for this behaviour.