A radioisotope has a half-life of 4 days. How much of a 20-gram sample of this radioisotope remains at the end of each time period?

1) 4 days

2) 8 days

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1. half is gone after 4 days, so 1/2 is left or 10 grams
in the next 4 days, half of ten grams is lost so 5 grams is left

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2. Just a word of caution on the wording and me being pedantic. The half life is referring to the original isotope so I like to see the answer referring to this. The mass of the sample may have changed only very slightly.

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3. That's a VERY good point and one which I often ignore. Although I didn't respond to this query, I'm glad Dr Russ brought out this point because I think it IS ignored in most instances. And the reason it is ignored, I think, is that some actually don't realize this very point.

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