A compound decomposes with a first-order rate constant of $0.00854 {\mathrm{s}}^{-1}$. Calculate the concentration after 5.0 minutes for an initial concentration of 1.2M

General equation of first-order reaction, 

$A=A_0{e}^{-kt}$

${\mathrm{A}}_0$= Initial concentration of reactants 

$\mathrm{k}$ = rate constant, $\mathrm{t}=$ time taken 

$\mathrm{A}=$concentration of reactants after time  "t"

$A=1.2{\mathrm{e}}^{0.00854 \times 5 \times 60}$

$=1.2e^{-2.56}$

$\begin{array}{l}=1.2\times 0.07\\ =0.093 \mathrm{M}\end{array}$