Page 94 - Basic physical chemistry for the atmospheric sciences

80 Basic physical chemistry

.
Combining this with Eq. (4 2 2)
Kw = [H + (aq)]2

Therefore,
[H +(aq)] = [OH - (aq)] = vfK: (4.24)

and, at 25°C
[H + (aq)] = [OH - (aq)] = Yl . 00 x 1 0 - 14 = l . 00 x 1 0 - 7 M
H
From Exercise 4 . 8 we see that the ratio of + (aq) ions to H20(1)
molecules in pure water at 25°C is only

l . 00 x 1 0 - 7 M = l . 80 x 1 0 _ 9
55 .5 M
I n other wor s , there are only a few ions to every billion neutral water
d
s
molecule . This is why pure water is only a weak electrolyte (i. e . , its
electrical conductivity is very small).

Exercises
4.9. Answer, interpret, or explain the following in the light of the
principles presented in this chapter.
(a) Give examples of liquid solutions in which the solvent
is liquid and the solute is a gas, and in which the solvent
is liquid and the solute is solid.
(b) The heat released when oxygen dissolves in water is 1 3
kJ mo1 - 1 , and when nitrous oxide (N 0) dissolves in
2
• Assuming the randomness factor
water 20 kJ mol - 1
discussed in Section 4 . 3 is the same in both cases,
which gas do you think would have the higher solubility
in water?
(c) Soda pop goes "flat" if it warms up.
(d) If a glass of cold water warms up, bubbles of air form
s
on the inside of the gla s .
i
t
(e) F i s h need oxygen dissolved in water o survive. What s
the effect of heating rivers by industry ("thermal pollu­
tion") on fish?
(f) Ethylene glycol is used as an antifreeze.
d
i
(g) Calcium chloride s used to melt ice on roa s .
(h) Lemons freeze at a higher temperatures than oranges.