The heat of neutralisation of HCN with  NaOH is –12.1 kJ/mole, the energy of dissociation of HCN is

Since some heat is used up to dissociate weak acid, HCN
Let the enthalpy of ionization or dissociation of weak acid = x 
As we know that 
$\mathrm{\Delta H}=$ neutralisation of strong acid and strong base  = – 57.3 kJ / eq.
– 57.3 = –12.1 –x 
$\therefore =45.2\mathrm{kJ}/\mathrm{eq}.$