It's kind of baffling me when I'm encountering this question in this sub-chapter. It's just unusual. So I really need your help :D(adsbygoogle = window.adsbygoogle || []).push({});

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

If a rock sample was found to contain 1.16 × 10^{-7}mol of

argon-40, how much potassium-40 (t_{1/2}= 1.3 × 10^{9}yr)

would also have to be present for the rock to be 1.3 × 109

years old? See assumption in Problem 14.84.

And the problem 14.84 question is ...

A 500 mg sample of rock was found to have 2.45 × 10^{-6}

mol of potassium-40 (t_{1/2}= 1.3 × 109 yr) and 2.45 ×

10^{-6}mol of argon-40. How old was the rock? (Hint: What

assumption is made about the origin of the argon-

40?)

2. Relevant equations

k = In 2/t_{1/2}

3. The attempt at a solution

I just find out that the both K and Ar in periodic table have a closely enough molecular mass, which is 40 g/mol (39,1 for K and 39,95 for Ar). But it just weird when the molecular mass is multiplied with each moles of Ar and K to find mass, because it doesn't add up for 500 mg. Also I don't have any idea what does the t_{1/2}works for. Of course we could find the rate constant from the equation before for it.

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# Some Radiological Dating with Stoichiometry

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