2O₃(g) $\rightleftharpoons$ 3O₂(g)

At 300 K, ozone is fifty percent dissociated. The standard free energy change at this temperature and 1 atm pressure is ($-$) ____________ J mol^$-$1. (Nearest integer)

[Given : ln 1.35 = 0.3 and R = 8.3 J K^$-$1 mol^$-$1]