The rate constant for the first-order reaction
$$
\mathrm{N}_{2} \mathrm{O}_{5} \longrightarrow 2 \mathrm{NO}_{2}+\frac{1}{2} \mathrm{O}_{2}
$$
is $1.20 \times 10^{-2} \mathrm{~s}^{-1}$ at $45^{\circ} \mathrm{C},$ and the initial concentration of $\mathrm{N}_{2} \mathrm{O}_{5}$ is $0.01500 \mathrm{M}$. (a) How long will it take for the concentration to decrease to $0.00100 \mathrm{M}$ ? (b) How much longer will it take for a further decrease to $0.000900 \mathrm{M?}$