Please help - I can't figure this out...

For the reaction A --products, succesive half-lives are observed to be 10.0, 20.0 and 40.0 minutes for an experiment in which [A](original) = 0.10M. Calculate the concentraion of A at the following times:
a) 80.0 minutes
b) 30.0 minutes

According to my book (Zumdahl, 7th edition) the answers are a).011M and b) .025M

It seems to be that at 20 minutes the concentration shoult be .025M? I'm very lost - please explain how to solve the problem. thanks.

do your own homework, having people on here do your work for you isn't going to help you learn anything

well, that was very helpful. thanks. I'm not looking for someone to give me answers I'm looking for someone to help me understand the process -- it's called learning.

From a guy who calls himself livineasi...

Well, good luck with your "learning"

This thread is more appropriate in the DAT forum and can be found there. DP

For the reaction A --products, succesive half-lives are observed to be 10.0, 20.0 and 40.0 minutes for an experiment in which [A](original) = 0.10M. Calculate the concentraion of A at the following times:
a) 80.0 minutes
b) 30.0 minutes

According to my book (Zumdahl, 7th edition) the answers are a).011M and b) .025M

It seems to be that at 20 minutes the concentration shoult be .025M? I'm very lost - please explain how to solve the problem. thanks.

Between 20 and 40 minutes, or 30 minutes, you'll have two half lives, so your concentration is 0.025M

After 40 minutes you'll get three half lifes, so your concentration would be less then 0.0125...or 0.011M for Zumduhl. It isn't clear what the progression of half lifes are after 40 minutes...whether it is 40, 70, and 110...or 40, 80, and 120. Hope this helps.