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Chemistry - Isotopes

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The isotope caesium 137 has a half life of 30 years. How long will it take for the amount of this isotope in a sample of caesium to decay to one sixteenth of its original amount? Please can someone explain how I can calculate this from the quantities that have been stated? Thanks

It decays 1/2 each 30 years.
time amount left
30 1/2
60 1/4
90 1/8
see the pattern?

Mathematically, it can be written as

amountleft/originalamount= (1/2)time/halflifetime

or in logs
amount left/originalamount=e0.692time/halflifetime

Saved by Bob again! Thank you!

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