$$\mathrm{pH}$$ of a salt solution of weak acid $$(\mathrm{p} K_a=4)$$ and weak base $$(\mathrm{p} K_b=5)$$ at $$25^{\circ}$$ is

The solubility product of sparingly soluble salt $$A X_2$$ is $$3.2 \times 10^{-11}$$. Its solubility (in mol/L) is