Calculate ΔG° for the following reaction at 25°C.

a) 2Mg(s)+O₂(g) → 2MgO(s)

b) 2SO₂(g)+O₂(g) → 2SO₃(g)

c) 2C₂H₆(g)+7O₂(g) → 4CO₂(g)+6H₂O(l)