60 mL of a mixture of equal volume of Cl2 and an oxide of chlorine was heated and then cooled back to the original temperature. The resulting gas mixture was found to have volume of 75 ml. On treatment with caustic soda solution, the volume contracted to 15 ml. Assume that all the measurements are made at same T and P. Deduce the simplest formula for oxide of Cl2. The oxide of Cl2 on heating decomposes quantitatively to O2 and Cl2.