Page 115 - Basic physical chemistry for the atmospheric sciences

A c ids and bases I O I

2
in 1 µg of H S04 is 1 0 - 6 /96, where 96 is the molecular weight of
0
0
H2S04• Since this amount of H2S04 dissolves in . 5 0 g (or . 5 0 x 10 '
L) of cloud water
1 0 - 6 1 0
[H2SOil)J = 96 x 0.50 x _ 3 2.0 x 1 0 - 5 M

Therefore,
5
=
pH of cloud water = - log(4.0 x 1 0 - ) 4 .4
This illustrates that even in clean, remote environments the pH of
cloud water (and therefore rain) is potentially quite acidic. Of course,
some of this acidity will be offset by the absorption of ammonia into
cloud water, although the concentration of ammonia is often low over
the oceans. On the other hand, as we will see in Chapter 6, additional
sulfate may be produced in cloud drops. The likely potential range of
variability of the pH of cloud water and rain over the oceans is illus­
trated by Exercise (5 . 1 9).

Exercises
5 . 7 . Answer, interpret, or explain the following in the light of the
pr n ciples presented in this chapter.
i
(a) The conjugated bases of strong acids are weak bases ;
the conjugated bases of weak acids are strong bases .
(b) A solution f NaN02(aq) is basic.
o
(c) It can generally be assumed that a solution with an
)
-
excess of Cl ( aq) , SO� - ( aq , HC03(aq) etc. with re­
2
spect to Na + ( aq , K + ( aq , Mg + ( aq) etc. is acidic.
)
)
(d) Human blood is a complex aqueous solution that i s
continually replaced, b u t t h e pH o f t h e blood of a
healthy person does not vary much from 7.4.
(e) A solution has an optimal buffering capacity when con­
jugate acid-base pairs are present in equal concentra­
tions.
(f) The addition of calcium carbonate (CaC0 ) to small
3
lakes can help counteract the effects of acid rain.
(g) Seawater is a b u ffered solution (with a pH between . 1
8
and 8 .3 ) . What do you think are its dominant buffers
against acids and bases ? Where do these buffers orig­
inate?