6.7: Weak Acid or Weak Base Equilibria: Foundations of General Chemistry with Lab-2021- Schiren.pdf

8/7/21, 10 : 24 AM 6.7: Weak Acid or Weak Base Equilibria: Foundations of General Chemistry with Lab-2021- Schiren
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6.7: Weak Acid or Weak Base Equilibria
There are many weak acids and weak bases that ionize only partially by equilibrium reactions.
One of the most common weak acids is acetic acid whose structural formula is shown below:
The molecular formula, HC H O , is usually used for acetic acid when writing the equilibrium
ionization reaction:
The equilibrium constant expression for the dissociation, K , is written as follows
HC H O dissociates to an extent of about 1% in water, so we know that [H O] is a constant and
can be ignored in the expression producing the following known as K , the acid ionization constant
in which we also simplify [H O ] as [H ]:
The value of K is determined experimentally by measuring the pH of a solution of HC H O of
known starting molarity. This is used to calculate [H ] from which [C H O ], and the equilibrium
[HC H O ] can be determined. In the case of [HC H O ], K = 1.8 x 10 . The K of some common
weak acids is shown in the table below:
Acid Formula K
Acetic acid HC H O 1.8 X 10
Benzoic acid HC H O 6.3 X 10
2 3 2
c
2 3 2 2
a
3
+ +
a 2 3 2
+
2 3 2
-
2 3 2 2 3 2 a
-5
a
a
2 3 2
-5
7 5 2
-5

8/7/21, 10 : 24 AM 6.7: Weak Acid or Weak Base Equilibria: Foundations of General Chemistry with Lab-2021- Schiren
Benzoic acid HC H O 6.3 X 10
Cyanic acid HOCN 3.5 X 10
Formic acid HCHO 1.7 X 10
Hydrofluoric acid HF 6.8 X 10
Hypochlorous acid HOCl 3.5 X 10
Nitrous acid HNO 4.5 X 10
Propionic acid HC H O 1.3 X 10
Pyruvic acid HC H O 1.4 X 10
Examples:
Determine the value of K for acetic acid (HC H O ), if the pH of an 0.135 M solution is 2.81.
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