Page 114 - Basic physical chemistry for the atmospheric sciences

1 00 Basic physical chemistry

=
Finally, substituting values into Eq. (5.40) gives [Co5 - ( aq)] 5 . 0
x 1 0 - 1 1 M . 6
We can now derive the pH of the above system from
pH = - log[H 0 3 + (aq)] = - log( l . 8 x 1 0 -6) = 5 . 7

Therefore, rainwater (or any other water) that is exposed only to
atmospheric C02 at 25°C and l atm will have a pH of about . .
7
5
Of course, neither rainwater nor any other water is generally ex­
posed to C02 alone. Even in regions of the globe wel l removed from
sources of anthropogenic pollution, rainwater is exposed to natural
S02 gas and sulfate particles in the air, which can decrease the pH of
the rain to below 5 . 7 . In the absence of bases, such as NH3 (the main
atmosphere gaseous base) and CaC0 3 (from soil dusts), which are
often in low concentrations in natural air, the pH of rainwater can
vary from about 4.5 to 5 . 6 (with an average value of about 5) due
solely to variability in the sulfur content of the air.
I n regions of the globe where the concentrations of sulfur oxides and
nitrogen oxides are unusually high, due primarily to the combustion of
l
fossil fue s , rainwater consists of a mixture of sulfuric acid, nitric acid,
and water as well as other chemica s . In such circumstance , the pH
s
l
of rainwater can reach values of 4 or lower. This is the phenomenon
of a cid rain, which can cause damage to fish, soil, cro s , and property.
p
It also has important consequences for geochemical cycling of various
minerals and their constituent elements.
Exercise . 6 . The concentration of sulfate in the air over the remote
5
oceans is about l µ,g per cubic meter of air. Assuming that this sulfate
is sulfuric acid and that in a cloud all of it is dissolved in the cloud
drops , what will be the pH of the water in a cloud that contains . 5 0 g
0
of liquid water in a cubic meter of air? Ignore the effects of other
chemical .
s
i
Solution . H2S0 4 i s a strong acid that is 00 1 % ionized n water
H2S0il) + 2H20(1) P SO� - (aq) + 2H30 + (aq)
Therefore, 2 moles of H30 + (aq) are formed for every l mole of
H2S0 4 (1) that dissolves in water. Hence, the pH of the water is given
by

Therefore, to solve the problem we need to find the molar concentra­
tions of H2S04 that dissolves in the cloud water. The number of moles