The K for the neutralization reaction

NaOH(aq) + HCl(aq) ---> NaCl(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) + H⁺(aq) + Cl^-(aq) ---> Na⁺(aq) + Cl^-(aq) + H₂O(aq)

The ionic equation is;

OH^-(aq) + H⁺(aq) ---> H₂O(aq)

K for this reaction is 1/K_w or 1.0 x 10¹⁴. This K is very large so any reaction between a strong acid and a strong base goes completely to the right.