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if a 46 gram sample of a radioisotope, lead - 189 has a half life of 20 days then how many grams of the sample will remain after 60 days?

5.75 g.

Explanation:

It is known that the decay of isotopes of Pb-189 obeys first order kinetics. Also, it is clear that in first order decay the half-life time is independent of the initial concentration. The half life time of Pb-189 decay = 20 days.Thus, 60 hours represent (60/20) 3 half-lives.

∴ 46 g →(first half-life) 23 g →(second half-life) 11.5 g →(third half-life) 5.75 g.

So, The amount remained is 5.75 g after 3 half-lives (60 days).

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