16. Ions and Equilibrium;
       Acids and Bases
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       Weak Acids and Bases

Acetic acid is an organic acid, but behaves in a similar way:

Because acetic acid is so common, it is often given the shorthand representation HOAc, with Ac- symbolizing

The dissociation is then written

The acid-dissociation equilibrium constant is

In such expressions concentrations of ions and molecules are always given in units of molarity (M), or moles per liter of solution. The units of Ka are unmistakable, and ordinarily are omitted.


Ammonia is an example of a weak base that reacts with water to yield ammonium and hydroxide ions, but the reaction is far from complete:

Water is both solvent and reactant in this reaction. For moderately dilute solutions of ammonia, the concentration of water will not change appreciably from that of pure water:

For simplicity, the constant H2O concentration of 55.5 moles per liter can be brought to the left side of the equation and incoroporated into the equilibrium constant:

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