Question

1. A groundwater sample obtained near a septic tank has a pH of 6.90 and HCO₃^- concentration of 8.25 mmol L^-1. The source of bicarbonate is from the carbon dioxide produced from microbes and the process can be presented as the following chemical equation.

CO₂ (g) + 2H₂O (l) ⇌ HCO₃^- (aq) + H₃O⁺ (aq)    Ka = 4.5x10^-7

Provided with a Henry’s constant of CO₂ as 0.034 M/atm, answer the following questions.

1. Use an ICE table and work backward to determine the concentration of carbon dioxide present in the soil before it dissociates into bicarbonate.   (3 points)
2. determine the partial pressure of carbon dioxide (in atm) in the associated soil atmosphere. (3 points)
3. As a project manager, you suspect there is a leak from septic tank and you also know there are sulfur and nitrogen species present in this water sample. Use the pE-pH or E-pH diagram given in question #2 and predict the dominating species from this groundwater sample. Note: the pE value is purposely not given but use your judgement to decide in this case. (4 points)

Dominating nitrogen species in this water sample is    ___________________ (give the name of the species to practice naming)

Dominating sulfur species in this water sample is    ___________________ (give the name of the species to practice naming)

Answer 1

Part (a).

Ka1 = [H⁺][HCO₃^-]/[CO2]

i.e. 4.5*10^-7 = [H⁺]²/[CO2]

i.e. 4.5*10^-7 = (10^-6.90)²/[CO2]

i.e. [CO2] = 3.522*10^-8 M

Part (b).

According to Henry's law:

[CO2] = K_H * P_CO2

i.e. 3.522*10^-8 M = 0.034 M/atm * P_CO2

Therefore, P_CO2 = 1.036*10^-6 atm