(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Hi!

I'm stuck on this one too.

"A rock contains the radioactive isotope Rb-87. A piece of this rock contained 2,05*10^20 Rb-87 atoms and 8,25*10^20 decay products per kg. The half-life of Rb-87 is 47*10^9 years. How old is the rock?"

2. Relevant equations

[tex]N(t)=N_0(\frac{1}{2})^{t/T_{1/2}} [/tex]

3. The attempt at a solution

No nuclei (or atoms) disappear, they only decay. Therefore, the initial number of Rb-87 atoms must have been 2,05*10^20+8,25*10^20 per kg. Let's assume that we have a sample of 1 kg (the mass will change with decay, but not the number of atoms, so it doesn't matter).

Now, let x be its age. Then we can write

2,05*10^20=(2,05*10^20+8,25*10^20)(1/2)^(t/T_{1/2}) which gives us that x is approximately 22,9*10^9 years. However, in the key it says 1,7*10^9 years. What's wrong here?

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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# Radioactivity 2

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