1)If the initial concentration of N2O5 is 0.020 M, what is the concentration of N2O5 after 5 half-lives?
2)How long will it take for the N2O5 concentration to fall to 12.5% of its initial value?

please help me!! I'm so lost with all this:confused:

1) for each half-life, you have half of the substance remaining, so after n half lives, you have 0.5^n of the original substance remaining.

2) reversing the above reasoning, you need to solve 0.5^n = 0.125 for n. This is the number of half lives that must pass for 12.5% of the original substance to be left.

Let us know if you need any further help, and let us know specifically where you are having problems.

Thanks! But where does concentration tie into #1. For #2 I got -0.1803 for n, but I still don't know how I'm to find time. Is there a formula I could use? I don't mean to sound dumb, but I really have a hard time understanding this.

simply put n in as 5, and you will get the percent of the original substance that is left. So you just multiply yor original concentration by that percentage to get the concentration after 5 half lives.

for 2, I think you're doing something wrong. The answer is a positive integer. You might find it easier to solve it analytically.
After one half life you have 0.5 of the original left, after 2 half lives you have 0.5*0.5 = 0.25 of the original left, after 3 you multiply by 0.5 again, try it and you should get your answer.

doesd that make it any easier to understand?

Yes, thank you!! it does make it easier.