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# Chemistry Exam Question Help watch

1. Potassium-40 is a radioactive isotope that decays by beta emission and has a half life of 1.25x10^9 years.

Calculate how long it will take for the activity of the isotope to decay to 1/8th of it's original activity.
2. Well, think of it like this:

If we could quantify the isotopes original radioactivity we could give it the number 8.
Each 1.25x10^9 years this quantification (radioactivity) will half. We can therefore work out how many half lives are needed before the potassium-40 isotope is of '1' activity, and thus 1/8th of it's original activity.
3. For each half life, the radioactivity decreases by 50%, so:

1 half life: half of the original value
2 half lives: quarter of the original value
3 half lives: one eighth of the original value

So we know it takes three half lives, all that's left to do is multiply 1.25x10^9 years by 3 and there's your answer!
4. (Original post by sortofmusician)
For each half life, the radioactivity decreases by 50%, so:

1 half life: half of the original value
2 half lives: quarter of the original value
3 half lives: one eighth of the original value

So we know it takes three half lives, all that's left to do is multiply 1.25x10^9 years by 3 and there's your answer!
It's so simple now! Thank you!
5. (Original post by AndrewKn0x)
It's so simple now! Thank you!
You're very welcome!

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Updated: February 3, 2016
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