Answer
$K_{a}$ of HCN is $4.8977$$\times$$10^{-10}$ or upon more simplification it is $4.90$$\times$$10^{-10}$
Work Step by Step
$pK_{a}$ = $-log(K_{a})$
$pK_{a}$ = $9.31$
$K_{a}$ = $antilog$ $(-pK_{a})$
$OR$
$K_{a}$ = $10^{(-pK_{a})}$
Upon calculating,
$antilog$ $(-9.31)$ = $4.8977$$\times$$10^{-10}$
