Question

How much of a 100 g sample of radon-222 will remain unchanged after 7.64 days?

Answer 1

The half-life of a substance describes the time in which it takes for half of the radioactive isotope to decay into another form. In your question, it says that the half life of radon-222 is 3.82 days; in a 100g sample of radon, 3.82 days pass and 50g of that isotope is left.

7.64 days is the equivalent to two half lives (7.64/3.82), so only half of half of 100%, or 25%, of the isotope would remain. The answer is D

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