The rate constant for a second-order reaction is 0.13 m-1 sec-1. if the initial concentration of reactant is 0.26 mol/l, it takes sec for the concentration to decrease to 0.13 mol/l.

The number  of seconds for  the concentration  to decrease  to  0.13 mol  for a second  order  reaction is  calculated  using  the below  formula

1/At  =-Kt  +1/Ao  where  At   is  the final concentration, Ao  is the   initial concentration.  t = time in second   while k =  constant

1/0.13 =-0.13 (t) + 1/0.26
like terms together

1/0.13-1/0.26 =-0.13 t
divide both side by 0.13

T =29.6 seconds