T50 of first -order reaction is 10 min. Starting with 10 mol L–1, rate after 20 min is

Initial concentration = 10 mol L–1 Conc. after 20 min (two half lives) = 2.5 mol L–1 1/2 1 1 1 0.693 0.693 Now,k t 10min or 0.0693min rate k reactant 0.0693 2.5molL min x x