Question

Benzo[a] pyrene, or B[a]p, a polycyclic aromatic hydrocarbon, is measured in a wastewater lagoon 2.5 hr after a release at a concentration of 3ppb. Half life of B[a]p is 1hr via direct sunlight exposure.

a.Based on its structure, explain why light exposure can affect B[a]p

b.If only direct photodegradation is the only degradation process occurring, what is the initial concentration of B[a]p in the lagoon? Assume a first order kinetics

Answer 1

Answer:

a) Benzo[a] pyrene structure

It is the polycyclic Aromatic compound. Due to the photochemical reaction (Photo degradation) the decomposition of the molecule takes place.

b) Released concentration = 3 ppb

Time for the released = 2.5 h

Half life (t_1/2) = 1 h

It is the First Order reaction

number of t_1/2 = Total time / half life

= 2.5 / 1

= 2.5

a....1 t_1/2......a/2 .......2t_1/2.....a/4.....2.5t_1/2.......a/6

Here a Assume as is the Intial concentration

According to the question the concentration after 2.5 h is 3 ppb

The Initial concentration is a/6 = 3 ppb

a = 6 x 3

a = 18 ppb.

Note:

According to the priciple also we can caluculate

K = 2.303/t log a /(a-x)

K = 2.303 / t_1/2  (from this we can caluculate the K value).

a = Initial concentration

x = final or released concentration.

By applying the values we can caluculate.