Get Answers to all your Questions

header-bg qa

A first order reaction has a specific reaction rate of 10-2 sec-1. How much time will it take for 20 g of the reactant to reduce to 5 g?

  • Option 1)

    238.6 sec

  • Option 2)

    138.6 sec

  • Option 3)

    346.5 sec

  • Option 4)

    693.0 sec


Answers (1)


As discussed 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}




\frac{t_{1}}{2} \:\:of \:\:the \:\:reaction=\frac{0.693}{K}


A=\frac{A_{o}}{2^{t/ t\frac{1}{2}}}

A_{o}= Initial concentration

A= Final concentration

\Rightarrow 2^{t/ t\frac{1}{2}}= \frac{A_{o}}{A}= \frac{20g}{5g}=4=2^{2}

\therefore \frac{t}{t \frac{1}{2}}=2\:\:\:\:or\:\:\:\:t=2t \frac{1}{2}=138.6 sec



Option 1)

238.6 sec

This option is incorrect.

Option 2)

138.6 sec

This option is correct.

Option 3)

346.5 sec

This option is incorrect.

Option 4)

693.0 sec

This option is incorrect.

Posted by


View full answer

NEET 2024 Most scoring concepts

    Just Study 32% of the NEET syllabus and Score up to 100% marks