# Isohydric solutions are the solutions having the same concentration of hydronium ion. If $0.2 \;M \;HA$ (monobasic acid) solution is isohydric with $4×10^{-4}\; M\; HCl$ solution, then $K_b$ for $A¯$ is
$\begin{array}{1 1} 8 \times 10^{-7} \\ 1.25 \times 10^{-8} \\ 1.25 \times 10^{-6} \\ 8 \times 10^{7} \end{array}$