# 8.3 Practice Problems

1. If $K_H$ is  $\pu{29.51 atm L mol-1}$ for $\ce{CO2}$ and the atmospheric $\ce{CO2}$ concentration is $\pu{0.004 atm}$, what is $\ce{CO2}$ concentration in water at equilibrium?
2. m-Xylene enters the groundwater system. The average concentration of m-Xylene in the groundwater is $\pu{100 mg L-1}$. If the groundwater at the top of the saturated zone is in equilibrium with air, calculate the concentration of m-Xylene in the air in $\pu{mol L-1}$. $K_H$ for m-Xylene is $0.43736$. **Answer**: $\pu{4.12e-4 mol L-1}$
3. It was determined that the adsorption of toluene onto sedimentary particles followed a linear relationship, with $K_d$ = $\pu{9.9 L kg−1}$. If the toluene concentration in the groundwater is $\pu{4.5 mg L−1}$, calculate the concentration of toluene sorbed onto the sedimentary particles in the aquifer. **Answer**: $\pu{446 mg kg−1}$
4. The first-order rate constant for the breakdown of toluene under aerobic conditions was $\pu{−0.54  d-1}$; under anaerobic conditions, the rate constant was $\pu{−0.002  d-1}$. Calculate the time required to degrade $\pu{90 \%}$ of the toluene under aerobic and anaerobic conditions. **Answer**: $\pu{2.7 d}$ and $\pu{1151 d}$
5. Assume an organic contaminant is introduced into an aquifer. Right after the spill, the concentration of the organic contaminant is $\pu{12 mg L−1}$. A measurement made $\pu{20 d}$ later reveals that the concentration of the contaminant has decreased to $\pu{1 mg L−1}$. Assuming that this is a first-order reaction, what is the half-life for this particular organic contaminant? **Answer**: $\pu{5.6 d}$
6. Lindane (a commonly used pesticide) is considered a persistent organic pollutant (POP) in the environment. It has $\log K_H = 3.94$ and $\log K_{ow} = 3.78$, and first-order rate constants for its loss from air, water, and soil/sediment are $\pu{0.1 d-1}$, $\pu{0.01 d-1}$, and $\pu{0.0006 d-1}$, respectively. Estimate the half-life of lindane loss from air, water, and soil/sediment.
7. In a particular system, the half-life of Lindane undergoing aerobic decomposition is $\pu{210 d}$. Determine the duration needed for $\pu{90 \%}$ of the Lindane to break down. **Answer**: $\pu{698 d}$