General Chemistry Principles, Patterns, and Applications, 2011

As we noted earlier, the concentration of water is essentially constant for all reactions in aqueous solution,
so [H2O] in Equation 16.15 can be incorporated into a new quantity, the acid ionization constant (K a), also
called the acid dissociation constant:
Equation 16.16
Ka = K[H2O =
] [ H 3O+][A-] [ HA]
Thus the numerical values of K and Ka differ by the concentration of water (55.3 M). Again, for simplicity,
H3O + can be written as H + in Equation 16.16. Keep in mind, though, that free H + does not exist in aqueous
solutions and that a proton is transferred to H2O in all acid ionization reactions to form H3O + . The larger
the Ka, the stronger the acid and the higher the H + concentration at equilibrium. [1] The values of Ka for a
number of common acids are given in Table 16.2 "Values of ".
Table 16.2 Values of Ka, pKa, Kb, and pKb for Selected Acids (HA) and Their Conjugate Bases (A − )
Acid HA K a pKa A − K b pKb
hydroiodic acid HI 2 × 10 9 −9.3 I − 5.5 × 10 −24 23.26
sulfuric acid (1)* H2SO4 1 × 10 2 −2.0 HSO4 − 1 × 10 −16 16.0
nitric acid HNO3 2.3 × 10 1 −1.37 NO3 − 4.3 × 10 −16 15.37
hydronium ion H3O + 1.0 0.00 H2O 1.0 × 10 −14 14.00
sulfuric acid (2)* HSO4 − 1.0 × 10 −2 1.99 SO4 2− 9.8 × 10 −13 12.01
hydrofluoric acid HF 6.3 × 10 −4 3.20 F − 1.6 × 10 −11 10.80
nitrous acid HNO2 5.6 × 10 −4 3.25 NO2 − 1.8 × 10 −11 10.75
formic acid HCO2H 1.78 × 10 −4 3.750 HCO2 − 5.6 × 10 −11 10.25
benzoic acid C6H5CO2H 6.3 × 10 −5 4.20 C6H5CO2 − 1.6 × 10 −10 9.80
acetic acid CH3CO2H 1.7 × 10 −5 4.76 CH3CO2 − 5.8 × 10 −10 9.24
pyridinium ion C5H5NH + 5.9 × 10 −6 5.23 C5H5N 1.7 × 10 −9 8.77
hypochlorous acid HOCl 4.0 × 10 −8 7.40 OCl − 2.5 × 10 −7 6.60
hydrocyanic acid HCN 6.2 × 10 −10 9.21 CN − 1.6 × 10 −5 4.79
ammonium ion NH4 + 5.6 × 10 −10 9.25 NH3 1.8 × 10 −5 4.75
water H2O 1.0 × 10 −14 14.00 OH − 1.00 0.00
acetylene C2H2 1 × 10 −26 26.0 HC2 − 1 × 10 12 −12.0
ammonia NH3 1 × 10 −35 35.0 NH2 − 1 × 10 21 −21.0
*The number in parentheses indicates the ionization step referred to for a polyprotic acid.
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