A first order reaction has a rate constant 1.25 × 10^-3 s^-1. How long will 5 g of this reactant take to reduce to 2.5 g? [ log 2=0 · 301, log 3=0 ·…

\[\begin{aligned} & \mathrm{k}=\frac{2.303}{\mathrm{t}} \log \frac{[\mathrm{R}] \text { o }}{[\mathrm{R}]} \\ & \begin{aligned} 1.25 \times 10^{-3}=\frac{2.303}{\mathrm{t}} \log \left(\frac{5}{2.5}\right) & \\ & \mathrm{t}=\frac{2.303}{1.25 \times 10^{-3}} \log 2 \\ & \mathrm{t}=\frac{2 \cdot 303 \times 0.301}{1.25 \times 10^{-3}} \\ \mathrm{t}=554.5 \mathrm{~s} & \text { or } 5.54 \times 10^{2} \mathrm{~s} \end{aligned} \end{aligned} \]