waec model questions vol1 2019 physics
Objective • Question 21
Question 21
WAEC_MODEL_QUESTIONS_VOL1
•
2019
A rock sample contains two uranium isotopes: \(^{238}\text{U}\) with a half‑life of \(4.5\times10^{9}\) years and \(^{235}\text{U}\) with a half‑life of \(7.04\times10^{8}\) years. Let the initial numbers of atoms be \(N_{0,238}\) and \(N_{0,235}\) respectively. After exactly \(1.0\times10^{9}\) years the measured activity of \(^{238}\text{U}\) is twice that of \(^{235}\text{U}\). Determine the original ratio \(N_{0,238}:N_{0,235}\). Give your answer as a decimal to two significant figures.
Answer Options
Correct Answer
A.
5.57
B.
6.8
C.
3.9
D.
4.3
Correct Answer
A
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