If you just want the mass of the HCl, you can find it since you know volume and concentration. Your volume is 100 mL and concentration 1.2 M thus the moles of HCl you have is simply .1 * 1.2. Multiplying the moles of HCl by its molar mass will yield the mass.
I'm not sure if this is actually what you were looking for though, so I apologize if it is not.
Best.