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      As with a K_a expression, a K_b expression has no variations other than the actual formulas of the conjugate acid and base.

      Every acid has a K_a, and its corresponding conjugate base has a K_b. The K_a and the K_b of a conjugate acid-base pair are related by the K_w — the ionization constant for water. For a conjugate acid-base pair, . In addition, you can use the following shortcut: . (Are you enjoying this little bit of math? Biochemistry has a little more math than organic chemistry, but not nearly as much as general chemistry, so hang in there!)

      The K_b for the acetate ion, the conjugate base of acetic acid, would be associated with the following equilibrium expression:

      The K_a for the ammonium ion, the conjugate acid of ammonia, would be associated with the following equilibrium expression:

      An acid may be capable of donating more than one hydrogen ion. A biologically important example of this type of acid is phosphoric acid (H₃PO₄), which is a triprotic acid (meaning it can donate three hydrogen ions, one at a time). The equilibria for this acid are

The subscripts are modified to indicate the loss of hydrogen 1, hydrogen 2, or hydrogen 3. The associated K_a expressions are all of the form

      Here’s the breakdown for each K_a:

       

       

       

The value for each successive equilibrium constant often is significantly lower than the preceding value. Table 2-2 illustrates some biologically important acids. You can refer to this table when working buffer problems or determining which acid is stronger.

TABLE 2-2 The K_a Values for Biologically Important Acids

Acid	Ka1	Ka2	Ka3
Acetic acid (CH3COOH)
Pyruvic acid (CH3COCOOH)
Lactic acid (CH3CHOHCOOH)
Succinic acid (HOOCCH2CH2COOH)
Carbonic acid (H2CO3)
Phosphoric acid (H3PO4)			Скачать книгу В начало < 9 10 11 12 13 14 15 16 17 18 > В конец