Page 84 - Basic physical chemistry for the atmospheric sciences

70 Basic physical chemistry

Therefore, the total number of moles in 100 kg of dry air is
2700 + 725 + 32.6 = 3460. Hence, from Eq. (4. 2 )
2700 8 . 0
l/J nitrogen = = 0. 780 = 7 %
3460
725
2
lf!oxygen = = 0 . 2 1 0 = 1 .0%
3460
32.6
I/Jargon = 3460 = 0.00942 = 0.942%

E x ercise 4.2. The apparent molecular weight of solution is defined
Ma
as
Total mass of a solution
=
Ma Total number of moles in solution
What is the apparent molecular weight of dry air? Use the assumptions
and values given in Exercise 4. 1 .
Solution. From the definition of Ma and the values calculated in
Exercise 4. 1 , the apparent molecular weight of dry air is
1 0 0 x 0 3
1
Ma 28.9 g mole - 1
3460

4.3 Factors affecting solubility
The tendency toward maximum randomness (entropy) causes a solid
to dissolve. On the other hand, the precipitation of a solid from a
solution lowers the energy of the system, which is also a favored
condition. Equilibrium is reached when the concentration of the sol­
utes in a solution s such that the driving forces of these two opposing
i
tendencies (randomness and minimum energy) are the same. The en­
ergy factor is measured by the change in heat content when 1 mole of
a solid dissolves; this is called the heat o f solution. An increase in
g
temperature always favors the more random state (e. . , the dissolving
process for a solid). Consequently, the solubilities of solids increase
with increasing temperature.
Since gases are more random than liquids, randomness (or entropy)
decreases as a gas dissolves in a liquid. Therefore, unlike soli s , the
d
tendency toward maximum randomness favors the gas phase rather
than the gas dissolving in a liquid. However, when a gas dissolves in a
liquid, heat is released, and this favors the dissolving process. As in