Answer:
Suppose that the concentration of the greenhouse gas CO2 continues to increase in
our atmosphere until it reaches 500 ppm. What effect would this increase have on the pH
of rainwater? How much would the pH increase or decrease?
The increase in CO2 could decrease the pH of rainwater (if other sources of acidity like
SO2 and acidic aerosol particles are minimal).
With the CO2 mixing ratio = 380 ppm, [CO2(aq)] = KHPCO2 = (0.0338 M/atm)(3.8 x 10-4
atm) = 1.28 x 10-5 M
Reaction: CO2 + H2O ↔ H+
+ HCO3
-
Ka1 = 4.45 x 10-7
Note: although we can not really use an ICE (initial change equilibrium) table because
CO2 will not be depleted from the gas phase and CO2 will be constant, we can expect
that [H+
] = [HCO3
-
] (assuming no other sources of H+
)
Ka1 = 4.45 x 10-7 = [H+
][HCO3
-
]/[CO2] = [H+
]
2
/1.28 x 10-5
[H+
] = [(4.45 x 10-7)(1.28 x 10-5)]0.5 = 2.39 x 10-6 M
pH = 5.62
With CO2 mixing ratio = 500 ppm, [CO2(aq)] = 1.69 x 10-5 M, and pH = 5.56 (so a
decrease of 0.06 units).
Explanation:
