Q 21 :    

$A\to B$

The molecule A changes into its isomeric form B by following a first order kinetics at a temperature of 1000 K. If the energy barrier with respect to reactant energy for such isomeric transformation is 191.48 kJ ${\mathrm{mol}}^{-1}$ and the frequency factor is ${10}^{20}$, the time required for 50% molecules of A to become B is ______ picoseconds (nearest integer).

[R = 8.314 J ${\text{K}}^{-1}{\text{mol}}^{-1}$]                 [2025]