A sample initially contains only U-238 isotope of uranium. With time, some of the U-238 radioactively decays into Pb-206 while the rest of it remains undisintegrated.
When the age of the sample is years, the ratio of mass of Pb-206 to that of U-238 in the sample is found to be 7. The value of P is _____.
[Given: Half-life of U-238 is years; ]
Official answer: Bonus: full marks to everyone
This question was dropped by the examiners, and marks were given to all. The working below shows why no single integer works.
Convert to a mole ratio. A mass ratio corresponds to
So .
Age.
so .
That is not an integer. If the ratio is read as an atom ratio instead (, three half-lives), . The ambiguity is why the question was dropped.