Compounds like {rm CCl_2F_2} are known as chlorofluorocarbons, or {rm CFCs}. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of {rm CCl_2F_2} is 289 {rm J}/{rm g}. What mass of this substance must evaporate in order to freeze 180 {rm g} of water initially at 20^circ {rm C}? (The heat of fusion of water is 334 {rm J/g}; the specific heat of water is 4.18 {rm J/g cdot K}.)