In a first order reaction the concentration of reactant decreases from 800 mol/dm3 to 50 mol/dm3 in 2 × 104 sec. The rate constant of reaction in sec–1 is:

2.303 log a k t a x At time t = 4 2 10 s x 3 3 800 50 a mol dm a x mol dm 4 4 1 1 2.303 800 log 50 2 10 2.303 log16 2 10 1.38 10 k k k s x x x Hence the correct answer is option (c)