Asample contains 16.75 g of the radioisotope u-236 and 50.25 g of its daughter isotope, th-232. how long did it take for decay to take place if one half-life of u-236 is 23 million years? 46 million years 69 million years 92 million years 115 million years

46 million years

Explanation:

The initial concentration of U-236 = the remaining concentration U-236 + the produced concentration of Th-232 = 16.75 g + 50.25 g = 67.0 g.

∵ U-236 has a half life of 23 million years.

67.0 grams of U-236 → (first half life = 23 million years) [33.5 grams of U-236  + 33.5 grams of Th-232] → (second half life = 23 million years) [16.75 grams of U-236  + 50.25 grams of Th-232].

So, it reaches the proposed concentrations after 2 half-lives = 2(23 million years) = 46 million years.

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