Diazonium salt decomposes as C6H5N2+Cl−→C6H5Cl+N2. At 0oC, the evolution of N2 becomes two times faster when the initial concentration of the salt is doubled. Therefore, it is

For reaction C6H5N2+Cl−→C6H5Cl+N2Let the rate law be     r=d[N2]dt=k[C6H5N2+Cl]x                                   =k[Salt]x                                  ..... (i)From given,        if  2r=2d[N2]dt=k2[salt]x .....(ii)Then, on dividing (i) by (ii), we get 12=(12)x⇒x=1∴ Reaction is of 1st order.