EK FL 2 #30

[sfsn]

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I really don't understand this question/explanation. Can anyone explain?

How long will it take for a 1 gram sample of carbon-11 to become 75% boron-11?

Answer: 40 minutes

Explanation:
According to the passage, carbon-11 has a half-life of 20 minutes and decays into boron-11. Therefore, after 2 half-lives, 75% of the carbon will have decayed into boron-11, and the remaining 25% will still be carbon-11. Also since boron-11 and carbon-11 have the same atomic mass, there is no need to consider proportion changing as mass of the atoms changes.

 

[theonlytycrane]

carbon-11 has a half-life of 20 minutes, so after 20 minutes half of it will have decayed (into boron-11).

At this point we have .5 g carbon-11 left. After 20 more minutes, we will have .25 g carbon-11 left because another half will have decayed (adding to the boron-11).

 

[sfsn]

Thanks, I don't know why I just couldn't figure out such a simple thing, even with the explanation. Also, they should explain it like you did. Much clearer and fewer words.