On decreasing the \( \mathrm{pH} \) from \( 7 \) to \( 2 \), the solubility of a sparingly soluble salt \( (\mathrm{MX}) \) of a weak acid \( (\mathrm{HX}) \) increased from \( 10^{-4} \, \mathrm{mol \, L^{-1}} \) to \( 10^{-3} \, \mathrm{mol \, L^{-1}} \). The \( \mathrm{p}K_a \) of \( \mathrm{HX} \) is:
