In a first order reaction the concentration of reactant decreases from 800 mol/dm3  to 50 mol/dm3 in 2×102sec⁡. The rate constant of reaction in sec-1 is

k=2.303tlog⁡10aa−x;  t=2×102, a=800,a−x=50k=2.3032×102log⁡1080050          =2.3032×102log⁡1016       =2.3032×102log⁡1024 =2.3032×102×4×0.301       =1.38×10−2s−1