Since the reaction is 2 nd order w.r.t CO. Thus, rate law is given as. r=k[CO]2 Let initial concentration of CO is a i.e. [CO]=a ∴r1=k(a)2=ka2 when concentration becomes doubled, i.e. [CO]=2a ∴r2=k(2a)2=4ka2 ∴r2=4r1 So, the rate of reaction becomes 4 times.