{title2=$K_a$}
{wiki}

The <acid dissociation constant> quantifies the <chemical equilibrium> for loss of one <proton> from a <chemical acid>. In <water>, write the actual proton transfer as $\mathrm{HA}+\mathrm H_2\mathrm O\rightleftharpoons\mathrm H_3\mathrm O^++\mathrm A^-$. Its full <dimensionless> <equilibrium constant> is
$$
K_{\mathrm{rxn}}=\frac{a_{\mathrm H_3\mathrm O^+}a_{\mathrm A^-}}{a_{\mathrm{HA}}a_{\mathrm H_2\mathrm O}}.
$$
The usual aqueous <acid dissociation constant> uses the <solvent> convention with <water> <thermodynamic activity> approximated by one:
$$
K_a=\frac{a_{\mathrm H^+}a_{\mathrm A^-}}{a_{\mathrm{HA}}},\qquad \mathrm pK_a=-\log_{10}K_a.
$$
Here hydrated <hydrogen ions> are understood; $\mathrm H^+$ is shorthand, not an isolated bare <proton> in the <chemical solution>. All <thermodynamic activities> and $K_a$ are <dimensionless quantities>, using consistent <standard states>. If <water> <thermodynamic activity> differs appreciably from one, retain it in the full <equilibrium constant> instead of silently mixing conventions.

With <activity coefficients> $\gamma_i$ and reference <concentration> $c^\circ=1\,\mathrm{mol\,L^{-1}}$, put $a_i=\gamma_i[i]/c^\circ$ for each <solute>. Then
$$
K_a=\frac{\gamma_{\mathrm H^+}\gamma_{\mathrm A^-}}{\gamma_{\mathrm{HA}}}
\frac{[\mathrm H^+][\mathrm A^-]}{c^\circ[\mathrm{HA}]}
\approx\frac{[\mathrm H^+][\mathrm A^-]}{c^\circ[\mathrm{HA}]}.
$$
The last expression is the dilute ideal <concentration> approximation, not the definition in a nonideal <chemical solution>. Omitting $c^\circ$ instead produces a quotient with units of <concentration>; its numerical value depends on the chosen units. Do not take a <logarithm> of that dimensional quotient. Conditional <concentration> quotients can change with <ionic strength> even at fixed <temperature>, whereas the thermodynamic <acid dissociation constant> retains the <activity coefficient> corrections. The <IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/acid dissociation constant> entry distinguishes these conventions.

At the same <temperature>, <solvent> and <standard states>, a larger <acid dissociation constant>, equivalently a smaller <pKa>, indicates a stronger tendency to donate a <proton>. It does not specify an <analytical concentration>, a <pH>, or a <reaction rate>. Composition and the <weak acid mass and charge balances> are needed to predict <pH>; <chemical kinetics> addresses how quickly the <chemical equilibrium> is approached.


 Acid dissociation constant (source code)