The dissociation constant of acetic acid is 0.000018 and that for cyanoacetic acid is 0.0036 at 298 — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

Isohydric solutions have the same $[\text{H}^+]$. For weak acids, $[\text{H}^+] \approx \sqrt{K_a C}$. Thus, $K_{a1}C_1 = K_{a2}C_2$. Since both contain equal moles ($n$), $C = n/V$, making $K_{a1}/V_1 = K_{a2}/V_2$. The volume ratio is $V_1/V_2 = K_{a1}/K_{a2} = 0.000018 / 0.0036 = 18 / 3600 = 1/200$.