Selected Values of Acid and Base Equilibrium Constants at 25 OC Ka
Acid
HNOdaqI H3OtIaq)
25
H20(1l
1.0 X TO-" 2.9 X
CH,OHIao)
1.0
Conjugate Bare
6
4.0 X 10P6 1.0 x 10-14 1.0
NOs-
~~0 OHCH-0-
34
molar concentrations and that of the water by 1.0 gives the usual approximate relationship
If the hase in question is OH-, the equilihrium is Equlllbrlum Constants for Water and Hydronlum Ion To the Editoc We agree with Starkey, Norman, and Hintze [1986, 63, 4731 that the acid equilibrium constants for water and hydronium ion should be included inany tabulation of acid and base strengths. However, we disagree with the numerical values that they assign to these constants. As pointed out by these authors, water is both the standard base and the standard acid aaainst which the streneths of solute acids and bases are measured when reported as'the conventional equilibrium constants K, and KI.. - In particular, for an acid, HAL in dilute aqueous sol"tion
the thermodynamic equilihrium constant is defined by the equilihrium value of the ratio 'AOHA
a ~ , ~ t
= K"
a
~
l
~
Conventionally, solute activities are referred to a 1.0 m standard state, and, for dilute solutions, these activities are frequently approximated by molar concentrations. The standard state for the solvent, however, is conventionally taken as the pure solvent. and. in a dilute solution. the solvent activiti is approx&atel~ 1.0. Using these approximations gives the familiar equilibrium constant expression
If the acid strength of H30+ is to he compared with that of other acids we must use the same conventions for it as for other acids. For the equilibrium system
the thermodynamic equilibrium constant is defined by
Whatever assumptions are made as to the values of these activities, this ratio must he 1.0 (as indeed i t must be for any exchange process). S~ecificallv,one cannot discriminate between
