[tex]\tt C.~Ka=\dfrac{[H^+][CN^-]}{[HCN]}[/tex]
Further explanation
According to Arrhenius, acids are substances which, when dissolved in water, release Hions.
An HₓY acid in water will ionize:
HₓY (aq) ⇒ xH⁺ (aq) + Yˣ- (aq)
In general, the weak acid ionization reaction
HA (aq) ⇒ H⁺ (aq) + A⁻ (aq)
Ka's value
[tex]\large {\boxed {\bold {Ka \: = \: \frac {[H ^ +] [A ^ -]} {[HA]}}}}[/tex]
So for HCN
HCN(aq)⇒H⁺(aq)+CN⁻(aq)
[tex]\tt Ka=\dfrac{[H^+][CN^-]}{[HCN]}[/tex]
