A first order reaction takes 40 minutes for 30% decomposition. Calculate t_1/2 for this reaction.

 (Given log 1.428 = 0.1548)

                A   -->  P

T=0         a          0

T=t       (a-x)       x

Now, it takes 40 min for 30% decomposition i.e. reactant left after 40 min is 70% of its initial concentration.

So,