+ - Water Contamination by Dichiobenil The herbicide dichlobenil (2,6-dichiorobe
ID: 885632 • Letter: #
Question
+ - Water Contamination by Dichiobenil The herbicide dichlobenil (2,6-dichiorobenzonitrile, C7H3C12N) has a solubility of 18.0 mg/L by mass in water at 20 C. The herbicide disintegrates with a half-life (first order) of 1 .00 year. Suppose that 317 kg of dichlobenil s applied in the fields surrounding a 7.50-acre pond having an average depth of 3.00 ft. Suppose further that half of the dichlobenil permeates into the pond and that this happens within a few days. Assume that the density of the pond water is 1.00 g/mL. The following conversion factors will be helpful: 1 acre = 43,560 ft^2 1 ft = 12 m 1 in = 2.54 cm 1 cm^3 = 1 mL 1 ft^3 = 28,317cm^3 Part A After a few days, what will be the concentration of dichlobenil in the pond? Express the concentration numerically in parts per million by mass. 5.71 ppm Part B How long would it take for the dichlobenil concentration in the pond to drop to 1.00 ppm following this single application? Express your answer numerically in years.Explanation / Answer
Solution :-
Initial concnetration = 5.71 ppm
half life = 1.00 year
how long it take to drop the concnetration to 1.00 ppm
The decomposition is the first order reaction
Lets first calculate the rate constant of the decay reaction
K= 0.693 / t1/2
K=0.693/1.00 yr
K= 0.693 yr-1
Now lets calculate the time required to drop the concnetration to 1.00 ppm
First order integrated rate law equation is as follows
ln([A]t /[A]o)= -k*t
[A]t= final concentration , [A]o =initial concentration , k=rate constant and t=time
lets put the values in the formula
ln[1.00/5.71] =-0.693 yr-1 * t
-1.7422 = -0.693 yr-1 * t
t= -1.7422 / 0.693 yr-1
t= 2.51 year
Therefore time needed to drop the concnetration to 1.00 ppm is 2.51 years
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