Answer
The chemical formula for the conjugate acid of $CN^-$ is:
$HCN$.
Work Step by Step
To find the conjugate acid of a base, you just need to add a proton $(H^+)$ to the compound.
1. Add one hydrogen.
$CN^- --\gt HCN^-$
2. Add "+1" to the charge of the compound:
$-1 -- \gt 0$
$HCN^- --\gt HCN$
