The reaction 2X to B is a zeroth order reaction. With initial [X] = 0.2 M, the half-life is 6 h. Starting from [X]_0 = 0.5 M, how long (in hours) does it take for [X] to fall to 0.2 M?

Home › MCQs › EEJ MAIN Chemistry › Question #5533

EEJ MAIN Chemistry QUESTION #5533

Question 1

The reaction $2X \to B$ is a zeroth order reaction. With initial $[X] = 0.2\ M$, the half-life is 6 h. Starting from $[X]_0 = 0.5\ M$, how long (in hours) does it take for $[X]$ to fall to 0.2 M?

9.0 h✔️
12.0 h
18.0 h
7.2 h

Correct Answer Explanation

For zeroth order: $t_{1/2} = \frac{[A]_0}{2k}$. Given $[X]_0=0.2\ M$ and $t_{1/2}=6\ h$: $k = \frac{0.2}{2\times6} = \frac{1}{60}\ M\,h^{-1}$. Now starting from $[X]_0=0.5\ M$, time to reach 0.2 M: $t = \frac{[A]_0 - [A]}{k} = \frac{0.5-0.2}{1/60} = 0.3\times60 = 18\ h$.

More Options