If 20.0 g of sulfur dioxide (SO₂) reacts with 10.0 g of oxygen (O₂) to produce sulfur trioxide (SO₃) according to the balanced equation 2SO₂(g) + O₂(g) → 2SO₃(g), what is the mass of the excess reactant remaining unreacted? (Molar masses: SO₂ = 64 g/mol, O₂ = 32 g/mol)