Which of the following cannot be determined experimentally.

Options

A. Order
B. Rate
C. Rate constant
D. Molecularity

Explanation

To solve the question of which of the following cannot be determined experimentally, we need to understand the concepts of molecularity and order in chemical kinetics.

### Step-by-Step Solution:

1. **Define Molecularity**:
- Molecularity refers to the number of reactant molecules involved in an elementary reaction step. It is a theoretical concept that can be determined by analyzing the reaction mechanism. For example, if a reaction involves two molecules colliding, its molecularity is 2.

2. **Define Order of Reaction**:
- The order of a reaction is determined experimentally and is defined as the sum of the powers of the concentration terms in the rate law expression. It can be a positive integer, zero, or even a fraction. The order reflects how the rate of reaction depends on the concentration of the reactants.

3. **Experimental Determination**:
- The order of a reaction can be determined using various experimental methods, such as the method of initial rates, integrated rate laws, or half-life methods. These methods involve measuring how changes in concentration affect the rate of reaction.

4. **Comparison of Molecularity and Order**:
- Molecularity is a fixed value based on the reaction mechanism and does not require experimental data to determine. In contrast, the order of a reaction is derived from experimental data and can vary depending on the conditions of the reaction.

5. **Conclusion**:
- Since molecularity can be inferred from the reaction mechanism without experimental data, while the order must be determined through experiments, the answer to the question is that **molecularity cannot be determined experimentally**.

### Final Answer:
The quantity that cannot be determined experimentally is **Molecularity**.

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