There are two stable isotopes of iridium, $\ce{^191Ir}$ and $\ce{^193Ir}$. If you randomly pick an iridium atom from a large collection of iridium atoms, which isotope are you more likely to select?

  • $\begingroup$ It depends on which isotope is more abundant. There should be tabulated values of these; a quick internet search will throw them up. $\endgroup$
    – user7232
    Commented Sep 17, 2014 at 11:57
  • $\begingroup$ even in the exams right ? $\endgroup$
    – shaima
    Commented Sep 17, 2014 at 12:13
  • $\begingroup$ Clearly not, but you'd expect such information to be given to you. $\endgroup$
    – user7232
    Commented Sep 17, 2014 at 12:38

1 Answer 1


An urn contains nothing but

  • 63 white balls and
  • 37 black balls.

You grab into the urn, pick one item, look at it and put it back.

What is the probability that you drew:

  • a ball
  • a white ball
  • a black ball
  • a red ball?
  • 1
    $\begingroup$ 1) 100% 2)63% 3)37% 4)0% $\endgroup$
    – shaima
    Commented Sep 17, 2014 at 11:28
  • $\begingroup$ @shaima Exactly :) Amazingly, 63% and 37% are roughly the abundancies of the two natural iridium isotopes: $\ce{^{193}Ir}$ and $\ce{^{191}Ir}$ $\endgroup$ Commented Sep 17, 2014 at 11:33

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