4-chlorobenzoic acid and 4-methoxyphenol are only very slightly soluble in water. The addition of NaHCO₃ to the water allows 4-chlorobenzoic acid to fully dissolve, but not 4-methoxyphenol. The addition of NaOH causes both 4-chlorobenzoic acid and 4-methoxyphenol to fully dissolve. Give an approximation for the pKₐ of 4-methoxyphenol. [The pKa of H₂CO₃ is 6.4.]