Question

# Rainwater is naturally slightly acidic due to the dissolved carbon dioxide. Acid rain results when acidic...

Rainwater is naturally slightly acidic due to the
dissolved carbon dioxide. Acid rain results when acidic sulfur
and nitrogen oxides produced during the combustion of coal
and oil react with rainwater (see Box 12.1).
(a) The partial pressure of CO2 in air saturated with water
vapor at 25 C and 1.00 atm is 3.04 104 atm. Henry’s
constant for CO2 in water is 2.3 102 molL1atm1; and,
for carbonic acid, pKa1 6.37. Assuming that all the dissolved
CO2 can be thought of as H2CO3, verify by calculation that the
pH of “normal” rainwater is about 5.7.
(b) Scientists investigating acid rain measured the pH of
a water sample from a lake and found it to be 4.8. The
total concentration of dissolved carbonates in the lake is
4.50 mmolL1. Determine the molar concentrations of the
carbonate species CO3
2, HCO3
, and H2CO3 in the lake.
(c) Suppose that 1.00 tonne (1 t 103 kg) of coal that is
2.5% sulfur by mass is burned in a coal-fi red plant. What mass
of SO2 is produced?
(d) What is the pH of rainwater when the SO2 generated in
part (c) dissolves in a volume of water equivalent to 2.0 cm
of rainfall over 2.6 km2? (The pKa1 of sulfurous acid is 1.81.
Consider the water to be initially pure and at a pH of 7.)
(e) If the SO2 in part (c) is fi rst oxidized to SO3 before the
rainfall, what would the pH of the same rainwater be?
(f) One process used to clean SO2 from the emissions of coalfi
red plants is to pass the stack gases along with air through a
wet calcium carbonate slurry, where the following reaction takes
place: CaCO3(s) SO2(g) O2(g) S CaSO4(s) CO2(g). What
mass of limestone (CaCO3) is needed to remove 50.0 kg of sulfur
dioxide from stack gases if the removal process is 90% effi cient?