The rate constant for a first order reaction is 4.606×10^-3 s^-1. The time required to reduce 2.0 g of the reactant to 0.2 g is :

1. 100 s

2. 200 s

3. 500 s

4. 1000 s

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Answer: 3. 500 s

For a first-order reaction $t=\frac{2.303}{k}\log\frac{[A]_0}{[A]}$.

$$t=\frac{2.303}{4.606\times10^{-3}}\log\frac{2.0}{0.2}=\frac{2.303}{4.606\times10^{-3}}\times1=500\ \mathrm{s}$$

Answer: option (3).

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