Uranium-238 decays to Lead-206. Both Rock X and Rock Y initially had a ratio of 100%. Uranium-238 to 0% Lead-206. Today, Rock X has a ratio of 50%. Uranium-238 to 50% Lead-206. Rock Y has a ratio of 25% Uranium-238 to 75% Lead-206. Which rock is older?

Question

Chemistry

Emilee Schmidt

4 May, 07:45

Uranium-238 decays to Lead-206. Both Rock X and Rock Y initially had a ratio of 100%. Uranium-238 to 0% Lead-206. Today, Rock X has a ratio of 50%. Uranium-238 to 50% Lead-206. Rock Y has a ratio of 25% Uranium-238 to 75% Lead-206. Which rock is older?

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Answers (1)

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Audrey Paul · Answer 1

Rock Y

Explanation:

The rock that is older shall be determined based on the number of half lives it has witnessed. A half life is simply the amount of time required for a sample to decay to half of its original mass.

Rock X

From 100% uranium, it decays to 50% uranium. This means it went through just one half life.

Rock Y

From 100% uranium to 25% uranium

100 - -> 50 (first half life)

50 - -> 25 (second half life)

This means it wen through 2 half lives.

Rock Y is older than Rock X