Your unknown CFC (chlorofluorocarbon) will contain 56.6% chlorine by mass.
The key to this problem is the fact that, after the decomposition reaction takes place, all the chlorine present in the CFC will now be chlorine gas.
This means that you can use the number of moles of chlorine gas the reaction formed to determine how many grams of chlorine the CFC contained.
To do this, use the ideal gas law equation
Use chlorine gas' molar mass to determine the number of grams produced
Since all the chlorine present in the CFC is now chlorine gas, the mass of chlorine your CFC contained is
To determine the percent composition by mass of chlorine in the CFC, simply divide the mass of chlorine by the total mass of the compound, and multiply by 100