# Chemistry Class 12 NCERT Solutions: Chapter 4 Chemical-Kinetic Part 9

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Q: 17. During nuclear explosion, one of the products is with half-life of years. If of was absorbed in the bones of a newly born baby instead of calcium, how much of it will remain after years and years if it is not lost metabolically.

Answer:

Here,

It is known that,

Therefore, of will remain after years.

Again,

Therefore, will remain after years

Q: 18. For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.

Answer:

For a first order reaction, the time required for 99% completion is

For a first order reaction, the time required for completion is

Therefore,

Hence, the time required for completion of a first order reaction is twice the time required for the completion of of the reaction.

Q: 19. A first order reaction takes decomposition. Calculate.

Answer:

For first order reaction,

Therefore, of the decomposition reaction is