The liquid phase reversible reaction $A\rightleftharpoons B$ is carried out in an isothermal $CSTR$ operating under steady state conditions. The inlet stream does not contain $B$ and the concentration of $A$ in the inlet stream is $10$ mol/lit. The concentrations of $A$ at the reactor exit, for residence times of $1\:s$ and $5\:s$ are $8$ mol/lit and $5$ mol/lit, respectively. Assume the forward and backward reactions are elementary following the first order rate law. Also assume that the system has constant molar density. The rate constant of the forward reaction (in $s^{-1}$, rounded off to the third decimal place) is _____________