The order of a reaction is determined from:

Options

A. The stoichiometric equation
B. Experimental rate equation
C. Number of reactant molecules
D. Number of product molecules

Answer

To determine the order of a reaction, we need to analyze the given options and understand the concept of reaction order in chemical kinetics.

### Step-by-Step Solution:

1. **Understand the Concept of Reaction Order**:
- The order of a reaction is defined as the sum of the powers of the concentration terms in the rate law expression for the reaction. It indicates how the rate of reaction depends on the concentration of reactants.

2. **Evaluate the Options**:
- **Option 1: Stoichiometric Equation**:
- The stoichiometric coefficients in a balanced chemical equation do not necessarily reflect the order of the reaction. Therefore, this option is not correct.
- **Option 2: Experimental Rate Equation**:
- The order of a reaction is determined experimentally by the rate law, which is derived from experimental data. This option is correct.
- **Option 3: Number of Reactant Molecules**:
- The number of reactant molecules does not directly determine the order of the reaction. This option is not correct.
- **Option 4: Number of Product Molecules**:
- Similar to option 3, the number of product molecules does not determine the order of the reaction. This option is also not correct.

3. **Conclusion**:
- The order of a reaction is determined from the experimental rate equation, which is derived from experimental observations and not merely from the stoichiometric coefficients or the number of reactant or product molecules.

### Final Answer:
The order of a reaction is determined from the **experimental rate equation**.