The reaction $2 X \rightarrow B$ is a zeroth order reaction. If the initial concentration of $X$ is $0.2 M$, the half life is 6 h . When the initial concentration of $X$ is 0.5 M , the time required to reach its final concentration of 0.2 M will be:

JEE Main · 2019 · 11 Jan 2019 · Shift 2 · Chemical Kinetics

The reaction

2 X \to B

is a zeroth order reaction. If the initial concentration of

X

0.2 M

, the half life is 6 h . When the initial concentration of

X

is 0.5 M , the time required to reach its final concentration of 0.2 M will be:

Answer: (C)

For a zero-order reaction the half-life is

t_{1/2} = \frac{[ X ] _{0}}{2 k}

. From

t_{1/2} = 6

h at

[X]_{0} = 0.2

k = \frac{0.2}{12} = \frac{1}{60}

M/h. Zero-order decay follows

[X]_{0} - [X]_{t} = k t

; taking the concentration from

0.5

M down to

0.2

0.5 - 0.2 = \frac{1}{60} t

, so

t = 0.3 \times 60 = 18

JEE Main · 2019 · 11 Jan 2019 · Shift 2 · Single correct option