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### Topic: Help with Basic Nuclear Chemistry  (Read 5326 times)

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#### viperman24

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##### Help with Basic Nuclear Chemistry
« on: April 28, 2010, 07:26:03 PM »
130I decays by emission of beta particles to form stable 130Xe. A 3.00 g iodine sample containing some I-130 was recorded as having 2464 disintegrations per min. k = 0.00094 min -1. How many radioactive I-130 atoms are present in the sample?

I'm thoroughly confused with this question. I started with the formula R=kN with R being the disintegrations per time, k being the decay constant and N being the number of radioactive nuclei (a bit confused on what N really means).

Plugging in what I have (R=2464, k=0.00094), I found "N" to be 2621276.596. From there, I'm not sure how to find the number of radioactive I-130 atoms present in the sample. I've tried incorporating Avogadro's number but I think that may be the wrong direction to go. I've also tried dividing the disintegration number by the amount of grams of the same present.

I've also tried using t1/2=ln2/k which just gave me the half life time which leads me to a dead end.

All the methods I've tried so far resulted in the wrong answer. Maybe I'm not understanding the question. Can someone shed some light into this?

#### viperman24

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##### Re: Help with Basic Nuclear Chemistry
« Reply #1 on: April 28, 2010, 07:30:59 PM »
What is the mass defect of 27Al in u? The mass of 1 atom of Al-27 is 26.9815 u.

I also need help with this question. I'm not sure where to start. I know the mass of 1 neutron is 1.008665 but I don't know how to use that. I know I need to add up the protons/neutrons/electrons in Al-27 and calculate the difference between that an 26.9815u (not sure what "u" means).

Extrapolating that idea, the calculation would be something like this:

Al has 13 protons => mass of proton x 13
Al has a mass number of 27 => 27-13 = 14 neutrons => mass of neutron x 14

The mass of an electron is too small to calculate, so it can be omitted.

Meaning I would have to find (mass proton x 13) + (mass neutron x 14) and find the difference between that and 26.9815. Can someone confirm this?

#### Borek

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##### Re: Help with Basic Nuclear Chemistry
« Reply #2 on: April 29, 2010, 02:52:51 AM »
Meaning I would have to find (mass proton x 13) + (mass neutron x 14) and find the difference between that and 26.9815. Can someone confirm this?

That's OK.
(a bit confused on what N really means).

Can you elaborate on your confusion?
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#### viperman24

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##### Re: Help with Basic Nuclear Chemistry
« Reply #3 on: April 29, 2010, 08:32:26 PM »
I guess what I mean is what exactly are radioactive nuclei and how does it relate to the problem?

Thanks for the help on the first problem.

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