Here's why:

* Rate Law: The rate law for the decomposition of N₂O₅ is:

Rate = k[N₂O₅]

where:

* Rate is the rate of the reaction

* k is the rate constant

* [N₂O₅] is the concentration of nitrogen pentoxide

* First-Order Dependence: The rate law shows that the rate of the reaction is directly proportional to the concentration of N₂O₅. This means that if you double the concentration of N₂O₅, the rate of the reaction will also double. This is the defining characteristic of a first-order reaction.

Experimental Evidence: The first-order nature of this reaction has been verified experimentally through various methods, including:

* Monitoring the change in concentration of N₂O₅ over time: The experimental data fits a first-order integrated rate law.

* Half-life: The half-life of the reaction is constant, regardless of the initial concentration of N₂O₅, which is another hallmark of a first-order reaction.