Which of the following statement is not correct ?

Options

A. Molecularity of a reaction cannot be fractional
B. Molecularity of a reaction cannot be more than three
C. Molecularity of a reaction may or may not be equal to the order of reaction
D. Molecularity of a reaction is obtained from balanced chemical equation

Answer

To determine which statement about molecularity is not correct, we will analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Understanding Molecularity**:
- Molecularity refers to the number of reactant molecules involved in an elementary reaction step. It is always a whole number and indicates how many molecules collide in a single step of the reaction.

2. **Analyzing Each Statement**:
- **Statement 1**: "Molecularity of a reaction cannot be fractional."
- This statement is **true**. Molecularity is defined as the number of molecules participating in a reaction, and since you cannot have a fraction of a molecule, this statement holds.

- **Statement 2**: "Molecularity of the reaction cannot be more than three."
- This statement is **true** for elementary reactions. While theoretically, molecularity can be greater than three, such reactions are extremely rare and often involve simultaneous collisions of more than three molecules, which is practically impossible. Thus, for practical purposes, this statement is considered true.

- **Statement 3**: "Molecularity of a reaction may or may not be equal to the order of reaction."
- This statement is **true**. The order of a reaction is determined experimentally and can be different from the molecularity, which is a theoretical concept based on the stoichiometry of the elementary steps. Therefore, this statement is correct.

- **Statement 4**: "Molecularity of a reaction is obtained from a balanced chemical equation."
- This statement is **not always true**. While molecularity can be inferred from the balanced equation for elementary reactions, for complex reactions, one must consider the reaction mechanism to determine the molecularity of each elementary step. Therefore, this statement is incorrect.

3. **Conclusion**:
- The statement that is not correct is: "Molecularity of a reaction is obtained from a balanced chemical equation."

### Summary of Findings:
- The correct answer to the question is that the fourth statement is not correct.