The STP stands for standard temperature and pressure which means in a condition where the temperature is 273.15K and the pressure is 0.986atm. In STP, an ideal gas will have a volume 22.4 liters for every mol of gas. Then, the amount of molecule of the 33.6 l of chlorine gas (Cl2) would be:
volume of gas/ number of mol= 22.4l/mol
33.6l/ number of mol= 22.4l/mol
number of mol= 33.6l / (22.4 liters/mol)= 1.5 moles
The mass of the gas would be: 1.5 moles * 70.906 g/mol= 106.359 grams