For a first order reaction,
$t_{1 / 2}=\frac{0.693}{k}$ ( $k=$ rate constant )
${\Rightarrow \quad 1} {386=\frac{0.693}{k}}$
${\Rightarrow \quad k} {=5 \times 10^{-4} \,\mathrm{s}^{-1}}$
${=0.5 \times 10^{-3}\, \mathrm{s}^{-1}}$
(આપેલ:$R =8.31\,JK ^{-1}\,mol ^{-1}$)
(આપેલ : $\ln 2=0.693)$