Which of the following representations of I (van't Hoff factor) is not correct ?

Options

A. `i="Observed colligative property"/"Expected colligative property"`
B. `i="Normal molecular mass "/"Observed molecular mass "`
C. `i="Number of molecules actually present "/"Number of molecules expected to be present "`
D. `i="Total number of particles taken before association/dissociation"/"Number of particles after association/dissociation"`

Answer

To determine which representation of the van't Hoff factor (I) is not correct, let's analyze each option step by step.

### Step 1: Understand the van't Hoff factor (I)
The van't Hoff factor (I) is a measure of the number of particles a solute produces in solution. It is defined as the ratio of the observed colligative property to the expected colligative property.

### Step 2: Analyze each option
1. **Option A**: Observed colligative property / Expected colligative property.
- This is a correct representation of the van't Hoff factor (I). It directly reflects how the colligative properties change based on the number of particles in solution.

2. **Option B**: Normal colligative molar mass / Observed molar mass.
- This is also a correct representation. The van't Hoff factor can be expressed in terms of molar mass changes due to dissociation or association of solute particles.

3. **Option C**: Number of molecules actually present / Number of molecules expected to be present.
- This is a correct representation as well. It captures the essence of how many particles are present in solution compared to what was initially added.

4. **Option D**: Total number of particles taken before association or dissociation / Number of particles after association or dissociation.
- This is an incorrect representation. The van't Hoff factor is defined as the total number of particles after dissociation divided by the number of particles initially added, not the other way around.

### Conclusion
The incorrect representation of the van't Hoff factor (I) is **Option D**.