Lafayette College
Department of Civil and Environmental Engineering
CE 321: Environmental Engineering and Science
Fall 2014
Homework #7
Due: Friday, 10/24/14
SOLUTIONS
1) Henry’s Law and pH calculations
a. Suppose the gas above the soda in a bottle of soft drink is pure CO2 at a pressure of 2 atm.
Calculate the [CO2] at 25 oC.
Henry’s Law = [CO2] = KH’Pgas
Where KH = 0.0333947 mol/(L·atm) @ 25 oC
b. Calculate the pH of the soft drink. Start with the following chemical reaction and realize
the solution will be somewhat acidic (negligible carbonate):
CO2 + H2O  H+ + HCO3(Hint: Assume the only cations and anions in the system are H+, OH- and HCO3-)
2) At 20oC the partial pressure of chloroform (CHCl3) in a storage tank is 18 mm of mercury.
Determine the equilibrium concentration of chloroform. Report your answer as mg/L of
chloroform.
Additional information: 760 mm of mercury = 1 atmosphere
H @ 20oC for Chloroform = 5.88 x 10-3 atm-1
3) By volume the concentration of oxygen in air is about 21 %. Find the equilibrium
concentration of O2 in water (mole/L and mg/L) at 25oC and 1 atm of pressure. Recalculate
it for Denver at an altitude of 1525m. (P = Po – 1.15*(10-4)*H; P = atmospheric pressure at
altitude H (atm), H = altitude (m), Po = atmospheric pressure at sea level (atm)).
4) Suppose the gas above the soda in a bottle of soft drink is pure CO2 at a pressure of 2 atm.
Calculate [CO2] at 15oC. Also report your answer as mg/L.
5) It has been estimated that the concentration of CO2 in the atmosphere before the Industrial
Revolution was about 275 ppm (0.000275 atm = 275 x 10-6 atm). If the accumulation of CO2
in the atmosphere continues, the by the middle of the next century it will probably be around
600 ppm (0.0006 atm = 600 x 10-6 atm). Calculate the pH of rainwater (neglecting the effect
of any other gases such as H2S and NO2) at 25oC in each of these times.