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here is a question says : enter image description here

I know that atom's number is found by = $6.022*10^{23} * mol$

but how this formula is used here is what i dont know

what's the $idea$ of this question?

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You need more information. They probably expect you to look in a book and see that the atomic weight (which is a weighted average over all isotopes) of $\ce{Tl}$ is 204.383. Now if x is the amount of $\ce{^203Tl}$ with atomic weight 202.97 and (1-x) is....

Does that get you started?

$$\ce{(x*202_{.}97) + ((1-x)*204_{.}97)~=~?}$$

Can you get to the answer from here?

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  • $\begingroup$ could you write one more line? $\endgroup$ – Maher Jun 20 '14 at 0:57
  • $\begingroup$ I think i got it. The final answer is 2.51, right? $\endgroup$ – Maher Jun 20 '14 at 20:38
  • $\begingroup$ Set the above equation equal to 204.383 and solve for x, the fractional amount of the TL 203 isotope. You'll find that x=29%. So about 29% of the 203 isotope and (1-.29) or 71% of the 205 isotope. $\endgroup$ – ron Jun 20 '14 at 21:38

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