O=C1C([N-]S(=O)(=O)c2cccs2)=C([n+]2ccccc2)C(=O)c2ccccc21
An acid's negative base-10 logarithmic dissociation constant (pka) follows directly from its underlying thermodynamics; pKa is directly proportional to its standard Gibbs free energy change. What is the pka value of this molecule?