The K for the neutralization reaction

NaOH(aq) + HA(aq) ---> NaA(aq) + H₂O(aq)

can be determined, but we need the net ionic equation to see how we can determine K.

The ionic equation is;

Na⁺(aq) + OH^-(aq) + HA(aq) ---> Na⁺(aq) + A^-(aq) + H₂O(aq)

Since HA is a weak acid it does not completely dissociate into ions. The ionic equation is;

OH^-(aq) + HA(aq) ---> H₂O(aq) + A^-(aq)

K for this reaction is K_a/K_w or 1.0 x 10^-5/1.0 x 10¹⁴ or 1.0 x 10⁹. This K is very large so any reaction between a weak acid and a strong base also goes completely to the right.