Initially 2 moles of CO are present. At equilibrium, 1 mole of CO is present Hence, 2−1=1 moles of CO has reacted. 1 mole of CO will react with 1 mole of Cl2 to form 1 mole of COCl2. 3−1=2 moles of Cl2 remains at equilibrium The equilibrium constant Kc=[CO][Cl2][COCl2]=5 L1 mol×5 L2 mol5 L1 mol=2.5