Page 117 - Basic physical chemistry for the atmospheric sciences

Acids and bases 103

5 . 1 8 . One-tenth liter of a buffer solution contains hydrocyanic
acid, HCN(aq), and its con u gate base, cN - (aq) . The solu­
j
tion has a pH of 7.9 and contains 7 . 0 x 1 0 - 4 moles of
HCN(aq). (a) What is the molar concentration of CN - ( aq)
in the buffer solution? (b) By how much would the pH of
the solution change if I . O x 1 0 - 5 moles of HCI04 were
added? (c) B y how much would the pH of the solution
change if I . O x 10 - 5 moles of Na H were adde ? (d) What
O
d
is the total buffering capacity for acids of the original solu­
tion? The acid-dissociation constant for HCN is 4.8 x 1 0 - 1 0 .
5 . 1 9 . Assuming that in oceanic air sulfate concentrations can
range from 0.040 to 1 . 0 µ,g m - 3 , and cloud liquid water
, calculate the range
contents range from 0. 1 0 to 2.5 g m - 3
of pH values of cloud water over the oceans. Assume that
the sulfate is H2S04 and neglect other effects on the acidity.
Do you think that the upper pH value you have calculated
would be achieved in nature? If not, why?

Notes
The term alkaline is essentially a synonym for basic; it refers to any solution
containing appreciable OH ( aq) or a substance that can form such a solution. Thu ,
-
s
s
a
alkalis are soluble strong base , such s NaOH and KOH . Alkali metal means any
metal of the group Na, K, Li, Rb, and Cs. In general, the oxides of metallic elements
are basic, and the oxides of nonmetallic elements are acidic.
1
2 The proton has a radius of about i o - 5 m. Other ions that have electrons associated
with them have radii of about 10 10 m.
3 Those familiar with the structure of atoms would expect from these definitions that
acids are materials in which the outer electron orbitals are not completely filled with
electrons, and bases are materials with electrons available for sharing. For example,
from the Lewis viewpoint , H+ is an acid because it has an empty orbital that can
accept a pair of electrons, and OH - is a base because it has pairs of electrons
available for sharing.
4 The symbol pH was introduced by a Danish chemist, S. S!llrensen; p stands for the
Danish word for power and H for hydrogen. With a change in sign, pH is the power
of ten of the hydrogen ion concentration in moles per liter.
5 Another way of viewing this is that hydrolysis is the reverse of acid dissociation [cf.
Reaction (5.20)]. Thus, the weaker the acid [e.g. , H C2HP in Reaction (5.20)] the
2
2
more difficult it is to remove a proton from it, and the easier it is for its anion or
conjugate base [e.g. , C2H30;(aq) in Reaction (5.20)) to attach a proton from water
(i . e . , to hydrolyze).
6 In calculations such as this, where several approximations are made, the solutions
should be checked by substituting the derived values back into the original equations
to see if reasonable equalities are obtained. This is left as an exercise for the reader
for this case.