### JEE Main On 16 April 2018 Question 20

##### Question: If 50 % of a reaction occurs in 100 second and 75% of the reaction occurs in 200 second, the order of this reaction is: [JEE Main 16-4-2018]

#### Options:

A) 1

B) Zero

C) 2

D) 3

## Show Answer

#### Answer:

Correct Answer: A

#### Solution:

50% of a reaction occurs in 100 second and 75% of the reaction occurs in 200 second,

the order of this reaction is 1. Let the initial reactant concentration is 1 M.

In first 100 seconds, reactant concentration is reduced to 50% or

$ 1\times \frac{50}{100}=0.5,M. $ In second 100 seconds (total 200 seconds from start),

reactant concentration is reduced to $ 100-75=25% $ or $ 1\times \frac{25}{100}=0.25,M. $

Thus we observe that after each 100 second period, the reactant concentration is reduced to one half.

Hence, 100 second is half-life period and is independent of reactant concentration.

This is characteristic of first order reaction.