Chemical Kinetics - Result Question 10

####11. The reaction, 2XB is a zeroth order reaction. If the initial concentration of X is 0.2M, the half-life is 6h. When the initial concentration of X is 0.5M, the time required to reach its final concentration of 0.2M will be ( 2019 Main, 11 Jan II)

(a) 7.2h

(b) 18.0h

(c) 12.0h

(d) 9.0h

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Answer:

Correct Answer: 11. (b)

Solution:

  1. For zero order reaction,

[A0][At]=kt

where, [A0]= initial concentration

[At]= final concentration at time ’ t '

k= rate constant

Also, for zero order reaction, t1/2=[A0]2k

Given, t1/2=6h and [A0]=0.2M

6=0.22k  or, k=0.22×6=160

Now, from Eq. (i)

[A0][At]=kt

Given, [A0]=0.5M,[At]=0.2M

$$ \begin{aligned} \therefore \quad 0.5-0.2 & =\frac{1}{60} \times t \quad\left[\because k=\frac{1}{60}\right] \



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