The rate constant for the reaction, $2 \mathrm{~N}_2 \mathrm{O}_{5(\mathrm{~g})} \rightarrow 2 \mathrm{~N}_2 \mathrm{O}_{4(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}$ is $4.98 \times 10^{-4} \mathrm{~s}^{-1}$. What is order of reaction?

The unit of the rate constant $k$ for a reaction of order $n$ is $[\text{concentration}]^{1-n} \text{time}^{-1}$.

Given $k = 4.98 \times 10^{-4} \text{s}^{-1}$, we equate the units:

$[\text{concentration}]^{1-n} \text{s}^{-1} = \text{s}^{-1}$.

For dimensional consistency, the concentration term must be dimensionless, requiring $1 - n = 0$.

Solving gives $n = 1$, indicating the reaction is first-order.

The correct option is C.