Answer
See the answer below.
Work Step by Step
The number of moles of salt:
$10.8\ g\div 49.01\ g/mol=0.22\ mol$
Molarity:
$0.22\ mol\div 0.5\ L=0.44\ M=[Na^+]$
Net ionic reaction:
$CN^-+H_2O\leftrightarrow HCN+OH^-$
$K=[HCN][OH^-]/[CN^-]=K_w/K_a$
$1\cdot10^{-14}\div4\cdot10^{-10}=x\cdot x/(0.44-x)$
$x=0.0033\ M=[HCN]=[OH^-]$
$[CN^-]=0.437\ M$
$[H_3O^+]=K_w/[OH^-]=3.03\cdot10^{-12}\ M$
