Page 50 - Soil and water contamination, 2nd edition

Basic environmental chemistry 37

Example 2.11 Weak acids
In a solution of acetic acid (CH COOH), the equilibrium concentrations are found to be
3
-
-1
-1
[CH COOH] = 10 mmol l and [CH COO ] = 0.422 mmol l . Calculate the pH of this
3 3
solution and the pK of acetic acid.
a
Solution
For the dissociation reaction of acetic acid
+
CH COOH ↔ H + CH COO -
3 3
-
+
H and CH COO are produced in equal amounts, thus
3
3
[H + ] = [CH COO ] . 0 422 10 mol l -1
3
-3
+
pH = – log[H ] = – log[0.422·10 ] = 3.375
The acidity constant K is

a
[H + ][CH COO ] . 0 ( 422 10 3 ) 2
K a 3 . 1 78 10 5
[CH 3 COOH ] 10 10 3
pK log(K ) log( . 1 78 10 5 ) . 4 75
a a
2.9.3 Bases
-
+
Conversely, the definition of a base is that it generates hydroxide ions (OH ) or takes up H
ions when dissolved in water forming its conjugate acid :
B H O OH BH (2.53a)
2
[BH ][OH ]
K (2.53b)
b
[B ]
+
where B = a base , BH = the conjugate acid , and K = the basicity constant . Analogous to the
b
definitions of weak and strong acids, a strong base is a base that dissociates completely (e.g.
sodium hydroxide: NaOH), and a weak base, a base that dissolves partially (e.g. magnesium
hydroxide: Mg(OH) ).
2
Bases neutralise acids in a neutralisation reaction, resulting in the production of water
and a dissolved salt:
HA B H O A BH (2.54)
2
In general, acid –base reactions proceed in the direction which yields the weaker acid and the
weaker base.

Example 2.12 Weak bases
0.1 mmol NH is dissolved in distilled water. Calculate the resulting pH given the pK =
3 b
4.75 for the reaction
+
NH + H O ↔ NH + OH -
3 2 4

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