2.5 mL of 0.4 M weak monoacidic base ($k_b = 1 \times 10^{-12}$ at 25$^\circ$C) is titrated with $\frac{15}{2}$ M HCl in water at 25$^\circ$C. The concentration of H$^+$ at equivalence point is ($K_w = 1 \times 10^{-14}$, at 25$^\circ$C)