16. Ions and Equilibrium;
       Acids and Bases
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       Acids with several dissociating protons

The pKa, values are especially convenient, because the pKa of a dissociation reaction is just the pH at which the undissociated and dissociated forms are present in equal amounts:

If the pH exactly equals the pKa for the acid under consideration, then

and A- and HA are present in equal amounts. If the pH falls below pKa (more acidic), then log10 [A-]/[HA] will be negative, the ratio [A-]/[HA] will be less than 1.00, and the HA form will be favoured.

In physical terms, if the pH is lowered by the addition of H+, some of the excess H+ combines with A- to make more of the undissociated acid. The dissociation equilibrium is shifted to the left.

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