The gas-phase reaction described is:2NO 2NO + OTo determine the rate of this reaction, we need to use the rate law equation:Rate = k [NO]^nwhere Rate is the reaction rate, k is the rate constant 2.0 x 10^-3 s^-1 , [NO] is the concentration of nitrogen dioxide 0.10 M , and n is the reaction order.Since the reaction order is not provided, we will assume it is a first-order reaction n = 1 . This is a common assumption for simple decomposition reactions. If more information is provided, the reaction order can be adjusted accordingly.Now, we can plug in the values into the rate law equation:Rate = 2.0 x 10^-3 s^-1 0.10 M ^1Rate = 2.0 x 10^-4 M/sThe rate of the gas-phase reaction at 450 K and 0.10 M concentration of nitrogen dioxide is 2.0 x 10^-4 M/s.