# Chemistry *URGENT*

posted by Anon

URGENT help!

3. A rock sample was dated using potassium-40, Neasyrenebt ubducated tgat 1/8 of the original parent isotope is left in the rock sample. How old is the rock?

5. When a sample of lava solidfied, it contained 28g of uranium-238. It that lava sample was later found to contain only 7g of uranium-238, how many years had passed since the lava solidified?

6. AFter 25 years, the number of radioactive cobalt atoms in a sample is reduced to 1/32 of the original count. What is the half-life of this isotope?

How would you find the answers to these questions?
TEll me the steps.

Thank you very much.

1. DrBob222

#3 has so many typos I can't translate it. Please repost.

#5. ln(No/N) = kt. I don't see k listed in the problem but it can be calculated from k = 0.693/t1/2. Thus, look up the half-life for U-238, substitute in the second equation to find k, then solve the first equation for t. No = 28 g. N = 7 g. The unit for t will be th same units used for the half-life.

#6. Use ln(No/N) = kt.I would make up a number (some convenient number such as 160) for No, then 1/32 x 160 will be N and you can solve for k since you are given t. Then k = 0.693/t1/2 and solve for t1/2.

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