the acid-dissociation constanstant for acetic acid

CH3COOH(aq)<->H+(aq)+CH3COO-(aq) is known to be 1.8X10^-5 at 25C

Caclulate G0f forCH3COO-(aq).
G0f=-396.56KJ/mol for CH3COOH (aq), 0 Kj/mol for H+
explain your answer