Question 18.2: In how many half-lives will 10.0 g of a radioactive nuclide ...

In how many half-lives will 10.0 g of a radioactive nuclide decay to less than 10% of its original value?

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We know that 10% of the original amount is 1.0 g. After the first halflife, half the original material remains and half has decayed (5.00 g). After the second half-life, one-fourth of the original material remains (i.e., one-half of 5.00 g). This progression continues, reducing the quantity remaining by half for each half-life that passes.

4 3 2 1 0 Half-lives
6.25% 12.50% 25% 50% 100% Percent remaining
0.625 g 1.25 g 2.50 g 5.00 g 10.0 g Amount remaining

Therefore the amount remaining will be less than 10% sometime between
the third and the fourth half-lives.

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