Chemistry
posted by Anonymous on .
Calculate the halflife (in s) of a firstorder reaction if the concentration of the reactant is 0.0655 M 18.4 s after the reaction starts and is 0.0173 M 41.2 s after the reaction starts.
I got 25.6
Suppose the halflife is 23.7 s for a first order reaction and the reactant concentration is 0.0709 M 60.1 s after the reaction starts. How many seconds after the start of the reaction does it take for the reactant concentration to decrease to 0.0154 M?
I got 79.05.
Any help is appreciated!

I did #1 as
ln(0.0655/0.0173) = kt, the
k  0.693/t_{1/2} and did not obtain 25.6 s.
Use ln(No/N) = kt for #2.
I did't obtain 79 s for that.
Post your work for both of these and I'll look for the error. 
So is it ln(.0655/.0173) and then the answer over .0693 which is k to get 1.24?

and how do you find k? because for the second problem i did ln (.0709/.0154) to get 1.527 which is obviously not the answer.