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In a first order reaction, the concentration of the reactant, decreases from 0.8 M to 0.4 M in 15 minutes. The time taken for the concentration to change from 0.1 M to 0.025 M is

  • Option 1)

    30 minutes

  • Option 2)

    15 minutes

  • Option 3)

    7.5 minutes

  • Option 4)

    60 minutes.


Answers (1)


As we learnt in 

First Order Reaction -

The rate of the reaction is proportional to the first power of the concentration of the reaction

- wherein


R    \rightarrow        P

a                 0

a-x             x



\frac{-dx}{dt}=K(a-x)  [differentiate rate law]

ln \:[\frac{a}{a-x}]=kt \:(Integrated rate law)

Unit of k=sec^{-1}






for next condition


t=30 min

Option 1)

30 minutes

Correct option

Option 2)

15 minutes

Incorrect option

Option 3)

7.5 minutes

Incorrect option

Option 4)

60 minutes.

Incorrect option

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