What is molar concentration of weak monobasic acid if dissociation constant is 5 $$\times$$ 10$$^{-8}$$ and undergoes 0.5% dissociation?

Equilibrium constant for a reaction is 20. What is the value of $$\Delta G^{\circ}$$ at $$300 \mathrm{~K} ?\left(R=8 \times 10^{-3} \mathrm{~kJ}\right)$$