$K_a$ for benzoic acid, $C_6{H}_5\frac{C}{O}OH,$ is $6\mathrm{.}30E-5.$

$K_a$ for hydrocyanic acid, $HCN,$ is $4\mathrm{.}00E-10.$

$K_a$ for hydrofluoric acid, HF$,$ is $7\mathrm{.}20$E$-4.$

What is the formula for the weakest conjugate base?

$K_a$ for hydrocyanic acid, $HCN,$ is $4\mathrm{.}00E-10.$

$K_a$ for formic acid, $H\frac{C}{O}OH,$ is $1\mathrm{.}80E-4.$

$K_a$ for benzoic acid, $C_6{H}_5\frac{C}{O}OH,$ is $6\mathrm{.}30E-5.$

What is the formula for the weakest conjugate base?