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Blazing goIITian

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26 Oct 2009 21:32:20 IST

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789

fiitjee AITS( kinetics)

None

40 atoms of radioactive element x (t half= 10 hours) r present.how many r likely to remain after 40 hours....

a. 1

b. 2.5

c. 5

d. 3

since radioactivity follows 1st order kinetics so t half should remain constant and by doing dat d ans. which u will get is 2.5........but atoms can't be in fraction (didn't notice dat during exam.....) so how to proceed in this question?

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Comments (10)

Yagyadutt Mishra

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26 Oct 2009 21:49:09 IST

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Hello!!!!!!!

I guess in question..it is talking about moles of atoms..i.e 40 moles are at present..

Half life is 10 hrs..

So after every 10 hr mole will be half..

So 40 hr means 4 half life..i.e amount will be 1/16 or can say 2.5 moles.

zzzzz

Blazing goIITian

Joined: 12 Apr 2008

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26 Oct 2009 22:35:22 IST

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dude its not talking about 40 moles .....its 40 atoms only........and answers are 1 , 3 and 5.

Arka Halder

Hot goIITian

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26 Oct 2009 22:40:36 IST

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as atoms cant be in fraction,the ans 2.5 is invalid.thus 1,3 and 5 can be the likely answers.

zzzzz

Blazing goIITian

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26 Oct 2009 22:42:24 IST

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but how u r getting 1 , 3 ,5 ?????u just can't say dat since they r not fractions its d right ans.........

Sagar Saxena

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26 Oct 2009 23:49:37 IST

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hiiiiiiii

ans should be 1 ,5 , 3 .

it should a multiple ans type quest

any how 2.5 cant be an answer !!!!!

wHaT tHe F

Blazing goIITian

Joined: 18 Aug 2009

Posts: 531

26 Oct 2009 23:57:13 IST

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sagar sir he is asking the reason for gaurav's answer ....he didnt ask ane 1 to REPOST /COPY his answer ..lolz ..it makes no sense by repeating an answer already posted ....it doesnt make sense to me atleast ...

Yagyadutt Mishra

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27 Oct 2009 00:17:17 IST

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kkk got it..

I didn't read the question care fully

It is likely to be. so 1 3 5 is possible........yup gr88 gr88 mai bhi negative kar deta

Abhishek Sharma

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Joined: 20 Sep 2009

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27 Oct 2009 21:21:41 IST

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answer is all except 2.5the reason for this is that half life is calculated by probability.if we have moles(which has quite a large quantity of particles), the approximation are well under the probable amount(which is ABOUT half)but, with only 40 atoms given, anything is possible, except 2.5 because there is nothing called half-atom ;)i am confident and 100% sure about the answer.i'm in fiitjee class XI, and our teacher asked this question when we were doing chemical kinetics

zzzzz

Blazing goIITian

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29 Oct 2009 16:41:08 IST

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yeah dude......i too am in fiitjee..........after AITS i also asked my teacher he said d same thing .....infact any ans. less than 40 (of course integer) will be right answers..........

edison

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29 Oct 2009 17:14:17 IST

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Radioactivity is a random phenomena and obeys poisson distribution, the no. of nuclei remaining after given time is just a probability and is average of several 1000's of trials. Thus if no. of moles are 10 and half life is 20hrs, then it does not mean that the no. of moles with reduce to 5 after 20hrs. Rather 5 is the most probable answer and is statistical average of infinite samples. The answer can be either 4 or 6 or someting else, but the important thing is the more you deviate form 5 the probability of that no. of nuclie (remaining undecayed) will diminish and reduce.

Now in your question the answer is 2.5, but since the number of atoms can not be fractional so the next answer lying close to 2.5 is 3

Thus most likely answer is 3 atoms

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