Hello, I have been working on this problem going on a week today. I was wondering if anyone can give me some pointer or hints.
The half-life of U-238 is 4.5*109 yrs. A sample of rock of mass 1.6g is found to produce 29 disintegration per second (dis/s). Assuming that all the radioactivity is due to U-238, find the percent by mass of U-238 in the rock.
This is what I have so far, I am not sure if I am doing this right, I am going on the few hints my prof gave me.
I found k constant doing 0.693/half-life which is 1.54*10-10yrs-1
I then changed that into s-1. I did my conversion:
(1/1.54*10-10yr-1)(1 year/365 days)(1 day/24 hours)(1 hour/60 seconds)
my prof suggested that I used Rate=kNt, so I tried to use it. I am finding Nt with having the Rate and k. This is when I ran started running into difficulty.
I am not sure whether do plug it in:
I then changed nuclei remaining into grams using avagando's number and 238g for 1 mole of U-238.
(3.596*10-15)(6.002*1023 moles U-238)(238g U-238)=5.136799696*1011grams
(2.338709677*1017)(6.002*1023 moles U-238)(238g U-238)=3.340790645*1043grams
These large answers I got above make no sense to me and seemed way too big.
I don't know where to go from here or if the way I did it up till now is correct. If someone can give me hints and steps to follow for this problem I would much appreciate...I can't learn by having someone just tell me the answer.
Thank you for looking at my post and if you have any idea, please let me know.