Obtaining activity coefficients of conjugate acids of some common carboxylic acid molecules
If no specific activity coefficient data are available, I would use the general formula for ionic compounds of small enough ion cincentations:

$$
\begin{align} 
I &amp;= \frac{1}{2}\sum_{{\rm i}=1}^{n} c_{\rm i}z_{\rm i}^{2} \\
\log{\gamma} &amp;= -0.509 . ( z_+^m . |z_-|^n )^{\frac{1}{m+n}}. \sqrt{I} \quad (*)\\
\end{align}
$$

$c_i$ is the ion concentration.
$z_i$ is the ion charge.
$I$ is the Ionic strength.
$\gamma$ is the activity ciefficient by the Debye-Hückel equation.
(*) - See the link for more info and more complex formulas for the medium $I$.