Chemistry order problem

Chemistry
Tutor: None Selected Time limit: 1 Day

It is very desirable that pesticides eventually decompose in the environment to give harmless products. Let us assume the half life for the first order decomposition of a certain pesticide is 10.2 years. If the present concentration of a pesticide in a lake is 3.00x10^-3 M, what was the concentration 4.00 years ago?

Sep 27th, 2015

Thank you for the opportunity to help you with your question!

Half life= 10.2 years, t1/2= 0.693/k, k=0.693/10.2= 0.068

k=1/t*ln{[A0]/[A]}, but [A0]=? , [A]= 3* 10^-3, and t=4 years, 

 0.068= 1/4*ln{[A0]/3*10^-3}, this gives initial concentration [A0] = 3.94*10^-3M   IMG_20150927_094710[1].jpg 

Please let me know if you need any clarification. I'm always happy to answer your questions.
Sep 27th, 2015

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