Today, the uranium found on Earth contains 0.720% 235U (with a half-life of 0.700 billion years) and 99.28% ²³⁸U (with a half-life of 4.50 billion years). At a time 2.20 billion years ago, what percent of the uranium on Earth was ²³⁸U (assuming that no other uranium isotopes were present)? A) 95.6% B) 2.18% C) 6.29% D) 8.68% E) 4.53%

Question

Quizplus · Accepted Answer

To find the percentage of 238U 2.20 billion years ago, we need to account for the decay of both isotopes over that time. Using the decay formula, we calculate the remaining percentages of both isotopes and find that 238U was approximately 95.6% of the total uranium 2.20 billion years ago.

Today, the Uranium Found on Earth Contains 0