Chemistry Problem Help

Question

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?

Answer 1

A = a₀(0.5)^t/h, where a₀ = original amount, t = time, h = half-life, A = current amount

 

3.0 x 10^-3 = a₀(0.5)^4/10.2

0.003 ≈ a₀(0.5)^0.392156863

0.003 ≈ a₀(0.761989560)

0.00393706181 ≈ a₀

 

The concentration was approximately 3.9 x 10^-3 M