Final answer:
The molecularity of the rate-determining step in a reaction mechanism is determined by the number of reactant molecules colliding during that step. If the first step, often the slowest, involves two molecules, it is bimolecular.
Step-by-step explanation:
The molecularity of the rate-determining step in the proposed mechanism is the number of reactant molecules that collide during that step. In the context of the reaction between carbon monoxide and nitrogen dioxide, we need to look at the experimental rate law and the mechanism to determine the molecularity of the rate-determining step. Suppose the first step is the rate-determining step, and this step involves two molecules reacting together, then its molecularity would be bimolecular. This is deduced from the fact that the overall rate law matches the rate law expected from a bimolecular elementary step. The overall reaction rate is controlled by this slowest step in the reaction mechanism.
For instance, if the first step involving carbon monoxide (CO) and nitrogen dioxide (NO2) was the rate-limiting step and is represented as CO + NO2 → products, this step would be bimolecular, as there are two reactant molecules colliding in this elementary step.