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The pKa of an acid is the negative logarithm of its acid dissociation constant. Just as pH can be used to describe the hydrogen ion concentration of a solution, pKa can be used to describe the dissociation constant of a weak acid. The higher the pKa of an acid, the weaker is the acid.

Table 12.7 repeats the weak acids listed in Table 12.4 and gives the pKa of each. Notice that the acids with larger ionization constants have smaller pKa's.

TABLE 12.7
The pKa of some weak acids
Weak acid Ka pKa
acetic acid 1.8 X 10-5 4.74
formic acid 1.8 X 10-4 3.74
nitrous acid 4.6 X 10-4 3.34
hydrocyanic acid 4.9 X 10-10 9.31
carbonic acid: Ka1 4.3 X 10-7 6.37
Ka2 5.6 X 10-11 10.25
phosphoric acid:Ka1 7.5 X 10-3 2.12
Ka2 6.2 X 10-8 7.21
Ka3 2.2 X 10-13 12.67
ammonium ion 5.5 X 10-10 9.26


For example, formic acid (Ka = 1.8 X 10-4) is a stronger acid than acetic acid (Ka = 1.8 X 10-5). The pKa of formic acid is 3.74, a smaller number than 4.74, the pKa of acetic acid. Notice too that, for the polyprotic acids, the pKa increases with each ionization. For example, the pKa for the first ionization of phosphroic acid:

H3PO4 H+ + H2PO4-      pKa = 2.12

is much smaller than that of the second ionization:

H2PO4- H+ + HPO42-     pKa = 7.21

Phosphoric acid is a much stronger acid and therefore much more completely ionized in solution than the dihydrogen phosphate ion.

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