Chemistry *URGENT*
posted by Anon .
URGENT help!
3. A rock sample was dated using potassium40, 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 uranium238. It that lava sample was later found to contain only 7g of uranium238, 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 halflife of this isotope?
How would you find the answers to these questions?
TEll me the steps.
Thank you very much.

#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/t_{1/2}. Thus, look up the halflife for U238, 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 halflife.
#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/t_{1/2} and solve for t_{1/2}.