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  What is a Pseudo Unimolecular Reaction? A pseudo unimolecular reaction is a chemical reaction that is actually a second-order reaction(or higher order) but behaves like a first-order reaction under specific experimental conditions.  Condition: This behavior occurs when one of the reactants involved in the reaction is present in a **very large excess** compared to the other reactant(s). **Why it Behaves as First-Order:** 1. Since one reactant is present in a huge excess, its concentration changes very little during the course of the reaction. 2. Because its concentration remains virtually **constant**, it can be mathematically absorbed into the rate constant ($k$). 3. This makes the overall reaction rate appear to depend only on the concentration of the *other*, limiting reactant, thus giving it the appearance of a first-order reaction. ---  Example: Acid Hydrolysis of an Ester The best-known example is the **acid-catalyzed hydrolysis of an ester** (like Ethyl Acetate...