For a zero order reaction, the time taken for $50\%$ completion is $100\text{ seconds}$. The time taken for $100\%$ completion is:

Solution

1. For a zero order reaction, $t=\frac{[\text{A}{]}_0-[\text{A}{]}_t}{k}$.
2. For $50\%$ completion, $$\begin{array}{rl}{t}_{50}& =\frac{0.5[\text{A}{]}_0}{k}\\ & =100\text{ s}\end{array}$$.
3. For $100\%$ completion, $t_{100}=\frac{[\text{A}{]}_0}{k}$.
4. Since $t_{100}=2\times t_{50}$, $$\begin{array}{rl}{t}_{100}& =2\times 100\\ & =200\text{ s}\end{array}$$.
5. Hence the answer is (B).