Isohydric solutions are the solutions having the same concentration of hydronium ion. If solution is — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

For the strong acid HCl, $[\text{H}^+] = 4 \times 10^{-4}\text{ M}$. Because HA is isohydric, it also has $[\text{H}^+] = 4 \times 10^{-4}\text{ M}$. The dissociation constant of the weak acid is $K_a = \frac{[\text{H}^+]^2}{C} = \frac{(4 \times 10^{-4})^2}{0.2} = \frac{16 \times 10^{-8}}{0.2} = 8 \times 10^{-7}$. The question asks for the $K_b$ of its conjugate base $\text{A}^-$, which is $K_b = \frac{K_w}{K_a} = \frac{10^{-14}}{8 \times 10^{-7}} = 1.25 \times 10^{-8}$.